fix: allow pb2 files to be included in the output of py_gapic_assembly_pkg

[parthea]

This PR is needed to unblock googleapis/python-api-common-protos#3.

Fixes #1854 🦕

To validate the changes,

1. Check out the code in this branch
2. In the WORKSPACE file of googleapis/googleapis, make the following change

#http_archive(
#    name = "gapic_generator_python",
#    strip_prefix = "gapic-generator-python-%s" % _gapic_generator_python_version,
#    urls = ["https://github.com/googleapis/gapic-generator-python/archive/v%s.zip" % _gapic_generator_python_version],
#)
local_repository (
  name = "gapic_generator_python",
  path = "<absolute path to your clone of gapic-generator-python>",
)

3. Add the following code to BUILD.bazel
4. Run bazel build //google/type:google-type-py

py_gapic_assembly_pkg(
    name = "google-type-py",
    deps = [
        ":calendar_period_py_proto",
    ],
)

5. Confirm that pb2 files appear in the tar file at <path to googleapis>/bazel-bin/google/type/google-type-py.tar.gz

(py31013) partheniou@partheniou-vm-3:~/git/googleapis$ bazel build //google/type:google-type-py
INFO: Analyzed target //google/type:google-type-py (1 packages loaded, 4 targets configured).
INFO: Found 1 target...
INFO: From Action google/type/google-type-py.tar.gz:
tar: google-type-py: file changed as we read it
/usr/local/google/home/partheniou/.cache/bazel/_bazel_partheniou/9063cec94a6a99d93035235cd3e5a885/sandbox/linux-sandbox/4/execroot/com_google_googleapis
Target //google/type:google-type-py up-to-date:
  bazel-bin/google/type/google-type-py.tar.gz
INFO: Elapsed time: 0.328s, Critical Path: 0.04s
INFO: 2 processes: 1 internal, 1 linux-sandbox.
INFO: Build completed successfully, 2 total actions
(py31013) partheniou@partheniou-vm-3:~/git/googleapis$ tar -tf bazel-bin/google/type/google-type-py.tar.gz
google-type-py/
google-type-py/calendar_period_pb2.py