bazel(python): no py_(proto | grpc)_library targets to depend on when dependency has a GAPIC

[noahdietz]

When a proto depends on a proto in another package, there is a chance that the python target dependencies of the dependent target will be generated with the wrong dependency targets when the dependency is a GAPIC. This is because when a package is a GAPIC, the only python targets generated are the py_gapic_library and py_gapic_assembly_pkg targets - there are no py_proto_library or py_grpc_library targets generated. However, a dependent package is generated with py_proto_library and py_grpc_library targets as the dependencies. It really should be depending on the py_gapic_assembly_pkg target instead, which contains not only GAPIC code but also the protobuf & grpc code.

I think a good solution would be for the non-GAPIC packages to be genreated with a py_gapic_assembly_pkg target with just the grpc & protobuf targets as the packaged sources. This way, we can change build_gen to always generate a dependency on this single target rather than having to determine which targets a dependency has to use.

[hkdevandla]

@software-dov, can you please triage this?
@vam-google (for vis).

[hkdevandla]

Note: This impacts only APIs with cross package dependencies and there are very few APIs that fall into this category. Still this needs to be fixed. Perhaps this could be part of the bazel robustification work.

@viacheslav-rostovtsev (for vis)

[noahdietz]

Downgrading to P3 as there aren't any urgent cases and this may not be feasible to do with existing infrastructure.