Fix dotty plugin

The dotty sbt-bridge module assumes that it's going to get a
URLClassLoader from which it can extract all of the classpath urls. That
doesn't work with the old wrapped classloader because its classpath was
empty. As a nasty workaround, I override the getURLs method, which is
where it gets the URLs from. After this change the
compiler-project/dotty-compiler-plugin test passes.

main-command/src/main/scala/sbt/internal/classpath/ClassLoaderCache.scala

@@ -3,7 +3,7 @@ package sbt.internal.classpath
 import java.io.File
 import java.lang.management.ManagementFactory
 import java.lang.ref.{ Reference, ReferenceQueue, SoftReference }
-import java.net.URLClassLoader
+import java.net.{ URL, URLClassLoader }
 import java.util.concurrent.atomic.AtomicInteger
 
 import sbt.internal.inc.classpath.{
@@ -107,7 +107,13 @@ private[sbt] class ClassLoaderCache(
     else ClassLoaderReference.apply
   private[this] val cleanupThread = new CleanupThread(ClassLoaderCache.threadID.getAndIncrement())
   private[this] val lock = new Object
+
   private class WrappedLoader(parent: ClassLoader) extends URLClassLoader(Array.empty, parent) {
+    // This is to make dotty work which extracts the URLs from the loader
+    override def getURLs: Array[URL] = parent match {
+      case u: URLClassLoader => u.getURLs
+      case _                 => Array.empty
+    }
     override def toString: String = s"WrappedLoader($parent)"
   }
   private def close(classLoader: ClassLoader): Unit = classLoader match {