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
diff --git a/python/python.go b/python/python.go
index 6c837a8..7d77ca7 100644
--- a/python/python.go
+++ b/python/python.go
@@ -197,6 +197,14 @@
 	return String(p.properties.Pkg_path)
 }
 
+// PkgPath is the "public" version of `getPkgPath` that is only available during bp2build
+func (p *PythonLibraryModule) PkgPath(ctx android.BazelConversionContext) *string {
+	if ctx.Config().BuildMode != android.Bp2build {
+		ctx.ModuleErrorf("PkgPath is only supported in bp2build mode")
+	}
+	return p.properties.Pkg_path
+}
+
 func (p *PythonLibraryModule) getBaseProperties() *BaseProperties {
 	return &p.properties
 }