Make protobufs respect pkg_path properly

Currently, python protobuf sources are generated as if
pkg_path didn't exist, but then are moved into the pkg_path
directory after being generated. This means they're generated
with import statements in them that don't include the pkg_path.
These import statements won't work at all when pkg_path is at
least 2 levels deep, but currently erroneously work with a 1
level deep pkg_path because we mistakenly add the top-level
modules in a soong-built python zip to the PYTHONPATH. We want
to remove those modules from the PYTHONPATH, so the generated
protobuf source files have to use the correct imports.

Since there are existing cases of code that needs to be updated,
guard this new behavior behind a flag, protos_respect_pkg_path.
We will set this to true on modules individually as we update
them, and then eventually change the default to true and remove
this flag.

Bug: 247578564
Test: m py_proto_pkg_path_test && out/host/linux-x86/nativetest64/py_proto_pkg_path_test/py_proto_pkg_path_test
Change-Id: I3695cf5521837da087592f2ad5350201035b7b0e