labm8/py/test: Move global conftest into //labm8/py/internal.

This moves the conftest.py file at the root of the WORKSPACE into a
module in //labm8/py/internal that is explicitly loaded as a plugin by
//labm8/py:test.

This is part of the ongoing process to eliminate conftest.py files
from the repo and to enable --noconftest arg in pytest.

github.com/ChrisCummins/phd/issues/70

Signed-off-by: format 2020.01.16 <github.com/ChrisCummins/format>

labm8/py/BUILD

@@ -709,7 +709,7 @@ py_library(
     visibility = ["//visibility:public"],
     deps = [
         ":app",
-        "//:conftest",
+        "//labm8/py/internal:pytest_plugin",
         "//third_party/py/pytest",
     ],
 )

labm8/py/internal/BUILD

@@ -55,3 +55,15 @@ python_proto_library(
     visibility = ["//labm8/py:__subpackages__"],
     deps = [":logging_pb"],
 )
+
+py_library(
+    name = "pytest_plugin",
+    testonly = 1,
+    srcs = ["pytest_plugin.py"],
+    visibility = ["//labm8/py:__subpackages__"],
+    deps = [
+        "//:build_info",
+        "//labm8/py:app",
+        "//third_party/py/pytest",
+    ],
+)