Disable ForkClassLoader when --thin is passed

ForkClassLoader injects resources from another ClassLoader. When --thin
is passed, it could have injected resources that should have been
blacklisted.

Alternatively, maybe we could have adde a whitelisting layer on top of
it too…

amm/interp/src/main/scala/ammonite/interp/script/SingleScriptCompiler.scala

@@ -43,7 +43,7 @@ class SingleScriptCompiler(
   }
 
   private val frame = {
-    val f = Frame.createInitial(initialClassLoader)
+    val f = Frame.createInitial(initialClassLoader, forking = false)
     f.addClasspath(dependencies.jars.map(_.toNIO.toUri.toURL))
     f.addPluginClasspath(dependencies.pluginJars.map(_.toNIO.toUri.toURL))
     for ((clsName, byteCode) <- dependencies.byteCode)

amm/repl/src/main/scala/ammonite/repl/Repl.scala

@@ -55,7 +55,9 @@ class Repl(input: InputStream,
     """
   }.mkString(newLine)
 
-  val frames = Ref(List(Frame.createInitial(initialClassLoader)))
+  val frames = Ref(List(
+    Frame.createInitial(initialClassLoader, forking = classPathWhitelist.isEmpty)
+  ))
 
   /**
     * The current line number of the REPL, used to make sure every snippet

amm/runtime/src/main/scala/ammonite/runtime/ClassLoaders.scala

@@ -62,16 +62,22 @@ class Frame(val classloader: SpecialClassLoader,
     usedEarlierDefinitions0 = usedEarlierDefinitions
 }
 object Frame{
-  def createInitial(baseClassLoader: ClassLoader = Thread.currentThread().getContextClassLoader) = {
+  def createInitial(
+    baseClassLoader: ClassLoader = Thread.currentThread().getContextClassLoader,
+    forking: Boolean = true
+  ) = {
 
     // *Try* to load the JVM source files and make them available as resources,
     // so that the `source` helper can navigate to the sources within the
     // Java standard library
 
     val likelyJdkSourceLocation = os.Path(System.getProperty("java.home"))/os.up/"src.zip"
     val hash = SpecialClassLoader.initialClasspathSignature(baseClassLoader)
+    val parent =
+      if (forking) new ForkClassLoader(baseClassLoader, getClass.getClassLoader)
+      else baseClassLoader
     def special = new SpecialClassLoader(
-      new ForkClassLoader(baseClassLoader, getClass.getClassLoader),
+      parent,
       hash,
       Set.empty,
       likelyJdkSourceLocation.wrapped.toUri.toURL

amm/src/main/scala/ammonite/Main.scala

@@ -154,7 +154,7 @@ case class Main(predefCode: String = "",
         errorStream,
         verboseOutput
       )
-      val frame = Frame.createInitial(initialClassLoader)
+      val frame = Frame.createInitial(initialClassLoader, forking = classPathWhitelist.isEmpty)
 
       val customPredefs = predefFileInfoOpt.toSeq ++ Seq(
         PredefInfo(Name("CodePredef"), predefCode, false, None)