Invalidate symbols for artifact classfiles, refactor classfile parser

No longer run the classfile parser on Scala generated classfiles that
don't have a Scala signature (module classes, inner classes, etc).

Various cleanups in the classfile parser, minimize the work performed
on Scala classfiles. Before, the attributes section was parsed twice:
once to find the ScalaSig attribute, the second time to find the
ScalaSignature in the RuntimeVisibleAnnotations. Now everything happens
in the first iteration.

Also fixes a bug in the backend: classes ending in `$` did not get a
ScalaSignature by mistake. They were filtered out by the name-based
test that is supposed to identify module classes.

Author: lrytz

src/compiler/scala/tools/nsc/backend/jvm/BCodeHelpers.scala

@@ -501,7 +501,7 @@ abstract class BCodeHelpers extends BCodeIdiomatic with BytecodeWriters {
      */
     def getAnnotPickle(jclassName: String, sym: Symbol): Option[AnnotationInfo] = {
       currentRun.symData get sym match {
-        case Some(pickle) if !nme.isModuleName(newTermName(jclassName)) =>
+        case Some(pickle) if !sym.isModuleClass => // pickles for module classes are in the companion / mirror class
           val scalaAnnot = {
             val sigBytes = ScalaSigBytes(pickle.bytes.take(pickle.writeIndex))
             AnnotationInfo(sigBytes.sigAnnot, Nil, (nme.bytes, sigBytes) :: Nil)