Bugfixes for proto_library (proto.include_dirs)

The fixes are
- Dedupe the dir list. Since we partition the proto srcs per pacakge and
  then iterate the map, a single include directory was being listed
  multiple times
- Drop target_compatible_with from these proto_library targets. The
  compatibility will be enforced at the top-level <lang>_proto_library.

Test: Added a unit test

Change-Id: Ia18c0f8c495585010fd4e372092afe80c5d8290c