makes well-known packages and package classes consistent with each other

As discovered in https://groups.google.com/forum/#!topic/scala-user/RckXE90LoXo,
RootClass.sourceModule and EmptyPackageClass.sourceModule used to incorrectly
return NoSymbol instead of RootModule and EmptyPackage. This is now fixed.
Merge pull request #3314 from xeno-by/topic/roots

makes well-known packages and package classes consistent with each other