Exclude transitive scala-library dependency

Scaladoc depends on scala-xml, etc, built with the previous stable
version of Scala. This isn't ideal, but we're stuck with this for
now.

We were relying on Ivy to evict the transitive dependency on the
old version of scala-library. However, SBT treats evictions of
scala-library as suspicious and emits a custom warning.

This commit explicitly excludes the transitive dependency so
as to avoid these warnings. It also makes the output of `show update`
a little easier to read.

build.sbt

@@ -50,10 +50,12 @@
  *     https://groups.google.com/d/topic/scala-internals/gp5JsM1E0Fo/discussion
  */
 
+val boostrapScalaVersion = "2.11.5"
+
 lazy val commonSettings = Seq[Setting[_]](
   organization := "org.scala-lang",
   version := "2.11.6-SNAPSHOT",
-  scalaVersion := "2.11.5",
+  scalaVersion := boostrapScalaVersion,
   // we don't cross build Scala itself
   crossPaths := false,
   // do not add Scala library jar as a dependency automatically
@@ -154,13 +156,13 @@ lazy val repl = configureAsSubproject(project).
   settings(disableDocsAndPublishingTasks: _*).
   dependsOn(compiler)
 
+def moduleDependency(name: String) =
+  // exclusion of the scala-library transitive dependency avoids eviction warnings during `update`.
+  "org.scala-lang.modules" %% name % "1.0.3" exclude("org.scala-lang", "scala-library")
+
 lazy val scaladoc = configureAsSubproject(project).
   settings(
-    libraryDependencies ++= Seq(
-      "org.scala-lang.modules" %% "scala-xml" % "1.0.3",
-      "org.scala-lang.modules" %% "scala-parser-combinators" % "1.0.3",
-      "org.scala-lang.modules" %% "scala-partest" % "1.0.5"
-    )
+    libraryDependencies ++= Seq("scala-xml", "scala-parser-combinators", "scala-partest").map(moduleDependency)
   ).
   settings(disableDocsAndPublishingTasks: _*).
   dependsOn(compiler)
@@ -179,12 +181,10 @@ lazy val asm = configureAsForkOfJavaProject(project)
 
 lazy val root = (project in file(".")).
   aggregate(library, forkjoin, reflect, compiler, asm, interactive, repl,
-    scaladoc, scalap).
-  // make the root project an aggragate-only
-  // we disable sbt's built-in Ivy plugin in the root project
-  // so it doesn't produce any artifact including not building
-  // an empty jar
-  disablePlugins(plugins.IvyPlugin)
+    scaladoc, scalap).settings(
+    scalaVersion := boostrapScalaVersion,
+    ivyScala := ivyScala.value map { _.copy(overrideScalaVersion = true) }
+  )
 
 /**
  * Configures passed project as a subproject (e.g. compiler or repl)