Support for the protobuf standard lib in gRPC buck tools

Required for building P4Runtime stuff.

Also:
- support in grpc_jar for building external protobuf files
- minor code refactoring/cleanups

Change-Id: I50c09f967cc9257366eb028d4ea1502767d8d4a0
diff --git a/incubator/protobuf-dependencies/BUCK b/incubator/protobuf-dependencies/BUCK
index 825a324..01222cd 100644
--- a/incubator/protobuf-dependencies/BUCK
+++ b/incubator/protobuf-dependencies/BUCK
@@ -1,3 +1,3 @@
 include_defs('//bucklets/grpc.bucklet')
 
-fetch_protoc_binary()
\ No newline at end of file
+fetch_protoc(version = '3.2.0')
\ No newline at end of file