Add basic support for building Java AppEngine WAR

This support is based on kchodorow@'s genrule to build the Bazel's dashboard
AppEngine and was tested with it.

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MOS_MIGRATED_REVID=97135934

tools/build_rules/appengine/BUILD

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+# A target to ensure the servlet-api is not linked in the webapp.
+java_library(
+    name = "javax.servlet.api",
+    neverlink = 1,
+    visibility = ["//visibility:public"],
+    exports = ["@javax-servlet-api//jar:jar"],
+)

tools/build_rules/appengine/README.md

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+# Java AppEngine Rules for Bazel
+
+## Overview
+
+These build rules are used for building
+[Java AppEngine](https://cloud.google.com/appengine/docs/java/)
+application with Bazel. It does not aim at general WebApplication
+support but can be easily modified to handle a standard WebApplication.
+
+* [Setup](#setup)
+* [Basic Example](#basic-example)
+* [Build Rule Reference](#reference)
+  * [`appengine_war`](#appengine_war)
+  * [`java_war`](#java_war)
+
+<a name="setup"></a>
+## Setup
+
+To be able to use the Java AppEngine rules, you must make the AppEngine SDK
+available to Bazel. The easiest way to do so is by copying the content of
+`appengine.WORKSPACE` to your workspace file.
+
+<a name="basic-example"></a>
+## Basic Example
+
+Suppose you have the following directory structure for a simple AppEngine
+application:
+
+```
+[workspace]/
+    WORKSPACE
+    hello_app/
+        BUILD
+        java/my/webapp/
+            TestServlet.java
+        webapp/
+            index.html
+        webapp/WEB-INF
+            web.xml
+            appengine-web.xml
+```
+
+Then, to build your webapp, your `hello_app/BUILD` can look like:
+
+```python
+load("/tools/build_rules/appengine/appengine", "appengine_war")
+
+java_library(
+    name = "mylib",
+    srcs = ["java/my/webapp/TestServlet.java"],
+    deps = [
+        "//external:appengine/java/api",
+        "//external:javax/servlet/api",
+    ],
+)
+
+appengine_war(
+    name = "myapp",
+    jars = [":mylib"],
+    data = glob(["webapp/**"]),
+    data_path = "webapp",
+)
+```
+
+For simplicity, you can use the `java_war` rule to build an app from source.
+Your `hello_app/BUILD` file would then look like:
+
+```python
+load("/tools/build_rules/appengine/appengine", "java_war")
+
+java_war(
+    name = "myapp",
+    srcs = ["java/my/webapp/TestServlet.java"],
+    data = glob(["webapp/**"]),
+    data_path = "webapp",
+    deps = [
+        "//external:appengine/java/api",
+        "//external:javax/servlet/api",
+    ],
+)
+```
+
+You can then build the application with `bazel build //hello_app:myapp` and
+run in it a development server with `bazel run //hello_app:myapp`. This will
+bind a test server on port 8080. If you wish to select another port,
+simply append the `--port=12345` to the command-line.
+
+<a name="reference"></a>
+## Build Rule Reference [reference]
+
+<a name="appengine_war"></a>
+### `appengine_war`
+
+`appengine_war(name, jars, data, data_path)`
+
+<table>
+  <thead>
+    <tr>
+      <th>Attribute</th>
+      <th>Description</th>
+    </tr>
+  </thead>
+  <tbody>
+    <tr>
+      <td><code>name</code></td>
+      <td>
+        <code>Name, required</code>
+        <p>A unique name for this rule.</p>
+      </td>
+    </tr>
+    <tr>
+      <td><code>jars</code></td>
+      <td>
+        <code>List of labels, required</code>
+        <p>
+          List of JAR files that will be uncompressed as the code for the
+          Web Application.
+        </p>
+        <p>
+          If it is a `java_library` or a `java_import`, the
+          JAR from the runtime classpath will be added in the `lib` directory
+          of the Web Application.
+        </p>
+      </td>
+    </tr>
+    <tr>
+      <td><code>data</code></td>
+      <td>
+        <code>List of files, optional</code>
+        <p>List of files used by the Web Application at runtime.</p>
+        <p>
+          This attribute can be used to specify the list of resources to
+          be included into the WAR file.
+        </p>
+      </td>
+    </tr>
+    <tr>
+      <td><code>data_path</code></td>
+      <td>
+        <code>String, optional</code>
+        <p>Root path of the data.</p>
+        <p>
+          The directory structure from the data is preserved inside the
+          WebApplication but a prefix path determined by `data_path`
+          is removed from the the directory structure. This path can
+          be absolute from the workspace root if starting with a `/` or
+          relative to the rule's directory. It is set to `.` by default.
+        </p>
+      </td>
+    </tr>
+  </tbody>
+</table>
+
+<a name="java_war">
+### `java_war`
+
+`java_war(name, data, data_path, **kwargs)`
+
+<table>
+  <thead>
+    <tr>
+      <th>Attribute</th>
+      <th>Description</th>
+    </tr>
+  </thead>
+  <tbody>
+    <tr>
+      <td><code>name</code></td>
+      <td>
+        <code>Name, required</code>
+        <p>A unique name for this rule.</p>
+      </td>
+    </tr>
+    <tr>
+      <td><code>data</code></td>
+      <td>
+        <code>List of labels, optional</code>
+        <p>List of files used by the Web Application at runtime.</p>
+        <p>Passed to the <a href="#appengine_war">appengine_war</a> rule.</p>
+      </td>
+    </tr>
+    <tr>
+      <td><code>data_path</code></td>
+      <td>
+        <code>String, optional</code>
+        <p>Root path of the data.</p>
+        <p>Passed to the <a href="#appengine_war">appengine_war</a> rule.</p>
+      </td>
+    </tr>
+    <tr>
+      <td><code>**kwargs</code></td>
+      <td>
+        <code>see <a href="http://bazel.io/docs/build-encyclopedia.html#java_library">java_library</a></code>
+        <p>
+          The other arguments of this rule will be passed to build a `java_library`
+          that will be passed in the `jar` arguments of a
+          <a href="#appengine_war">appengine_war</a> rule.
+        </p>
+      </td>
+    </tr>
+  </tbody>
+</table>

tools/build_rules/appengine/appengine.BUILD

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+# Copyright 2015 Google Inc. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# BUILD file to use the Java AppEngine SDK with a remote repository.
+java_import(
+    name = "jars",
+    jars = glob(["**/*.jar"]),
+    visibility = ["//visibility:public"],
+)
+
+java_import(
+    name = "api",
+    jars = ["appengine-java-sdk-1.9.23/lib/impl/appengine-api.jar"],
+    visibility = ["//visibility:public"],
+    neverlink = 1,
+)
+
+filegroup(
+    name = "sdk",
+    srcs = glob(["appengine-java-sdk-1.9.23/**"]),
+    visibility = ["//visibility:public"],
+    path = "appengine-java-sdk-1.9.23",
+)

tools/build_rules/appengine/appengine.WORKSPACE

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+# Copyright 2015 Google Inc. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# WORKSPACE file example to download Java AppEngine dependencies.
+new_http_archive(
+    name = "appengine-java",
+    url = "http://central.maven.org/maven2/com/google/appengine/appengine-java-sdk/1.9.23/appengine-java-sdk-1.9.23.zip",
+    sha256 = "05e667036e9ef4f999b829fc08f8e5395b33a5a3c30afa9919213088db2b2e89",
+    build_file = "tools/build_rules/appengine/appengine.BUILD",
+)
+
+bind(
+    name = "appengine/java/sdk",
+    actual = "@appengine-java//:sdk",
+)
+
+bind(
+    name = "appengine/java/api",
+    actual = "@appengine-java//:api",
+)
+
+bind(
+    name = "appengine/java/jars",
+    actual = "@appengine-java//:jars",
+)
+
+maven_jar(
+    name = "javax-servlet-api",
+    artifact = "javax.servlet:servlet-api:2.5",
+)
+
+bind(
+    name = "javax/servlet/api",
+    actual = "//tools/build_rules/appengine:javax.servlet.api",
+)