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

android/proto.go

@@ -281,6 +281,13 @@ func Bp2buildProtoProperties(ctx Bp2buildMutatorContext, m *ModuleBase, srcs baz
 				}
 			}
 
+			if p, ok := m.module.(PkgPathInterface); ok && p.PkgPath(ctx) != nil {
+				// python_library with pkg_path
+				// proto_library for this module should have the pkg_path as the import_prefix
+				attrs.Import_prefix = p.PkgPath(ctx)
+				attrs.Strip_import_prefix = proptools.StringPtr("")
+			}
+
 			tags := ApexAvailableTagsWithoutTestApexes(ctx.(TopDownMutatorContext), ctx.Module())
 
 			moduleDir := ctx.ModuleDir()
@@ -333,6 +340,11 @@ func Bp2buildProtoProperties(ctx Bp2buildMutatorContext, m *ModuleBase, srcs baz
 	return info, true
 }
 
+// PkgPathInterface is used as a type assertion in bp2build to get pkg_path property of python_library_host
+type PkgPathInterface interface {
+	PkgPath(ctx BazelConversionContext) *string
+}
+
 var (
 	protoIncludeDirGeneratedSuffix = ".include_dir_bp2build_generated_proto"
 	protoIncludeDirsBp2buildKey    = NewOnceKey("protoIncludeDirsBp2build")