Re-use metabuild scala instance layer

At some point I noticed that projects with no scala sources in the build
loaded significantly faster than projects that had even a single scala
file -- no matter how simple that file was. This didn't really make
sense to me because *.sbt files _do_ have to be compiled. I finally
realized that classloading was a likely bottle neck because *.sbt
files are compiled on the sbt classpath while *.scala files are compiled
with a different classloader generated by the classloader cache. It then
occurred to me that we could pre-fill the classloader cache with the
scala layer of the sbt metabuild classloader.

I found that compared to 1.3.0-M5, a project with a simple scala file in
the project directory loaded about 2 seconds faster after this change.
Even if there are no scala sources in the build.sbt, there is a similar
performance improvement for running "sbt compile", which I found exited
2-3 seconds faster after this change.

main-command/src/main/scala/sbt/State.scala

@@ -8,20 +8,14 @@
 package sbt
 
 import java.io.File
+import java.net.{ URL, URLClassLoader }
 import java.util.concurrent.Callable
 
 import sbt.internal.classpath.ClassLoaderCache
 import sbt.internal.inc.classpath.{ ClassLoaderCache => IncClassLoaderCache }
-import sbt.util.Logger
-import sbt.internal.util.{
-  AttributeKey,
-  AttributeMap,
-  ErrorHandling,
-  ExitHook,
-  ExitHooks,
-  GlobalLogging
-}
 import sbt.internal.util.complete.{ HistoryCommands, Parser }
+import sbt.internal.util._
+import sbt.util.Logger
 
 /**
  * Data structure representing all command execution information.
@@ -202,6 +196,10 @@ trait StateOps extends Any {
 }
 
 object State {
+  private class UncloseableURLLoader(cp: Seq[File], parent: ClassLoader)
+      extends URLClassLoader(Array.empty, parent) {
+    override def getURLs: Array[URL] = cp.map(_.toURI.toURL).toArray
+  }
 
   /** Indicates where command execution should resume after a failure.*/
   val FailureWall = BasicCommandStrings.FailureWall
@@ -344,6 +342,31 @@ object State {
     def initializeClassLoaderCache: State = {
       s.get(BasicKeys.extendedClassLoaderCache).foreach(_.close())
       val cache = newClassLoaderCache
+      s.configuration.provider.scalaProvider.loader match {
+        case null => // This can happen in scripted
+        case fullScalaLoader =>
+          val jars = s.configuration.provider.scalaProvider.jars
+          val (library, rest) = jars.partition(_.getName == "scala-library.jar")
+          library.toList match {
+            case l @ lj :: Nil =>
+              fullScalaLoader.getParent match {
+                case null => // This can happen for old launchers.
+                case libraryLoader =>
+                  cache.cachedCustomClassloader(l, () => new UncloseableURLLoader(l, libraryLoader))
+                  fullScalaLoader match {
+                    case u: URLClassLoader
+                        if u.getURLs
+                          .filterNot(_ == lj.toURI.toURL)
+                          .sameElements(rest.map(_.toURI.toURL)) =>
+                      cache.cachedCustomClassloader(
+                        jars.toList,
+                        () => new UncloseableURLLoader(jars, fullScalaLoader)
+                      )
+                    case _ =>
+                  }
+              }
+          }
+      }
       s.put(BasicKeys.extendedClassLoaderCache, cache)
         .put(BasicKeys.classLoaderCache, new IncClassLoaderCache(cache))
     }