Translate python_libray.pkg_path to proto.import_prefix

If a python_library uses a pkg_path foo/bar, then the proto srcs in
that libray need to import the dep .proto as foo/bar/proto.proto.

This behavior is restricted to python modules. To implement this is in
bp2build, this CL creates a new interface with a single method
`PkgPath`. Only python module structs implement this interface, and
this method is only available during bp2build

Test: Added a bp2build unit test
Test: TH

Change-Id: If8d207c0b321f75337a053795826b283a5eaaf46