Not sure the history for why there were two objc_library targets, but
given the one has an issue about not working, merge them into a single
target and an alias.

- Use the alias for :objectivec since that naming doesn't seem to follow
  the other targets *_library targets here.
- Update :protobuf_objc to be use for the working target and just inline
  the sources/headers lists so there is less indirection in the file.

Fixes #5284