This commit resolves #18331.
This commit resolves #18256.
This commit resolves... another TODO that apparently didn't have an
associated github issue.

We swap out pubref's implementation of py_proto_library with our own,
which more closely mirrors the interface of the internal
py_proto_library, taking the descriptor file output of a proto_library
rule as input.

One minor change in behavior was introduced for simplicity. When a
py_proto_library depends on a proto_library with a source proto file in
a subdirectory of the bazel package, the import module of the resultant
python library will reflect the package, *not* the full directory of the
proto file, including both the bazel package and the subdirectories, as
pubref did previously. This behavior also more closely mirrors google
internal behavior.

This commit also introduces a slightly more stringent bazel format
script. Buildifier on its own will not take care of long lines, but by
running yapf first, we end up with a more legible file. At the moment,
there is no sanity check associated with this formatter.

* Add a unit test
* Integrate with Bazel
* Polish README.md