Support proto modules with external references.

Bug: 236055697
Test: Manual testing and adding new unit tests.
Change-Id: I984c0ecb93f0023727a39a3af3921820337bf8c7
diff --git a/cc/proto.go b/cc/proto.go
index 8e6d5ed..cf5ed04 100644
--- a/cc/proto.go
+++ b/cc/proto.go
@@ -178,7 +178,7 @@
 	var ret bp2buildProtoDeps
 
 	protoInfo, ok := android.Bp2buildProtoProperties(ctx, &m.ModuleBase, protoSrcs)
-	if !ok {
+	if !ok || protoInfo.Proto_libs.IsEmpty() {
 		return ret
 	}
 
@@ -201,9 +201,8 @@
 	dep := android.BazelLabelForModuleDepSingle(ctx, depName)
 	ret.protoDep = &bazel.LabelAttribute{Value: &dep}
 
-	protoLabel := bazel.Label{Label: ":" + protoInfo.Name}
 	var protoAttrs protoAttributes
-	protoAttrs.Deps.SetValue(bazel.LabelList{Includes: []bazel.Label{protoLabel}})
+	protoAttrs.Deps.SetValue(protoInfo.Proto_libs)
 
 	name := m.Name() + suffix