Use named loader for ScalaLibrary

I'd already made a ScalaReflect loader and it makes sense to have a
ScalaLibraryClassLoader as well.

main/src/main/java/sbt/internal/ScalaLibraryClassLoader.java

@@ -0,0 +1,34 @@
+/*
+ * sbt
+ * Copyright 2011 - 2018, Lightbend, Inc.
+ * Copyright 2008 - 2010, Mark Harrah
+ * Licensed under Apache License 2.0 (see LICENSE)
+ */
+
+package sbt.internal;
+
+import java.net.URL;
+import java.net.URLClassLoader;
+
+final class ScalaLibraryClassLoader extends URLClassLoader {
+  static {
+    ClassLoader.registerAsParallelCapable();
+  }
+
+  private final URL[] jars;
+
+  ScalaLibraryClassLoader(final URL[] jars, final ClassLoader parent) {
+    super(jars, parent);
+    this.jars = jars;
+  }
+
+  @Override
+  public String toString() {
+    final StringBuilder builder = new StringBuilder();
+    for (int i = 0; i < jars.length; ++ i) {
+      builder.append(jars[i].toString());
+      if (i < jars.length - 2) builder.append(", ");
+    }
+    return "ScalaLibraryClassLoader(" + builder + " parent = " + getParent() + ")";
+  }
+}

main/src/main/scala/sbt/internal/ClassLoaders.scala

@@ -133,7 +133,13 @@ private[sbt] object ClassLoaders {
           case _: AllLibraryJars => true
           case _                 => false
         }
-        val scalaLibraryLayer = layer(si.libraryJars, interfaceLoader, cache, resources, tmp)
+        val scalaLibraryLayer = {
+          cache.apply(
+            si.libraryJars.map(j => j -> IO.getModifiedTimeOrZero(j)).toList,
+            interfaceLoader,
+            () => new ScalaLibraryClassLoader(si.libraryJars.map(_.toURI.toURL), interfaceLoader)
+          )
+        }
         val cpFiles = fullCP.map(_._1)
 
         val scalaReflectJar = allDependencies.collectFirst {