Test

This reverts commit 9b8ab1e.

This reverts commit fe30f15.

* Add py_proto_library

* Bump versions of rules_proto and protobuf

* Update documentation

* Bump rules_pkg version

plugin_output was wrong in case multiple repositories are involved
and/or _virtual_imports.

The code is taken from `cc_proto_library` and has been verified in
practice.

Backport py_proto_library

**Problem**
Currently rules_python injects the `@com_google_protobuf//:protobuf_python`
as the protobuf runtime of proto-generated `py_proto_library` targets.
This actually is inconvenient since protoc version may not match up with
pip protobuf runtime.
Often pip protobuf needs to go ahead of the protoc version to use
3rdparty libraries etc.

**Solution**
This changes the `proto_lang_toolchain` definition since as far as I can tell
rules_python doesn't expose the knob to customize this at build time
unlike Java does with `--proto_toolchain_for_java`.

Use pip_protobuf as runtime