pw_protobuf_compiler: Depend on Python package

When pw_proto_library Python packages are nested in another package,
have the .python subtarget wrap the package in which they're nested.
pw_proto_library targets originally disabled the .python subtarget when
the proto library was nested in another Python package. This made having
one pw_proto_library target depend on another difficult, and created
an error condition that didn't need to exist.

Change-Id: I53fe712d21f9a3d3100b0c3c395abc60fa2c666a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/47760
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>