feat(bazel): support generation with x-api deps

[noahdietz]

Support BUILD.bazel file generation with cross-API dependencies. Such a dependency is identified by the presence of a version in the import path e.g. google/cloud/foo/v1/bar.proto, which would normally be converted into a dependency label like google/cloud/foo/v1:bar_proto. However, generated BUILD.bazel files in googleapis have proto_library targets that represent the entire package rather than one target per-proto-file. So, the API package is parsed out of the import path and used as the replacement in the label e.g. google/cloud/foo/v1:foo_proto.

Note: This implementation generates a py_proto_library dependency that will not have been generated into the dependency package and must be added manually. This can be fixed when py_gapic_library targets can be used in their place (something @parthea has mentioned wanting to fix for this specific x-api dep scenario).

Fixes http://b/269312831.

[noahdietz]

Just a friendly bump on this @alexander-fenster and @parthea. Tony, I'd love your take on how this will impact/how this could aid Python GAPIC generation & the migration of py_proto_library to py_gapic_library targets.

[noahdietz]

@alexander-fenster could you review this when you have a chance please? Thanks

[parthea]

@noahdietz py_gapic can now be used as a dependency instead of py_proto for python. I found that the dependency needs to be imported with py_import first.

See cl/520334574 as an example

[noahdietz]

@noahdietz py_gapic can now be used as a dependency instead of py_proto for python. I found that the dependency needs to be imported with py_import first.

Wonderful! I will adjust this PR accordingly.

[noahdietz]

@parthea sounds like we can resolve #46 too in a way. I think I will do this in a separate CL and have this one gated on that.