Misc refactorings in KotlinBuiltIns

- delete fields which were supposed to be used for caching, but mistakenly were
  never used
- delete/inline unused methods and constants
- delete useless logic related to root package fragment: package fragment
  providers are only supposed to find packages they're aware of
Move module 'serialization' to core/
Add missing copyrights across the project
Don't render stack trace in a warning

These stack traces are confusing: they make it seem as if an exception was
thrown