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/grpc-dependencies/BUCK b/incubator/grpc-dependencies/BUCK
index 393524a..8006e27 100644
--- a/incubator/grpc-dependencies/BUCK
+++ b/incubator/grpc-dependencies/BUCK
@@ -1,3 +1,3 @@
 include_defs('//bucklets/grpc.bucklet')
 
-fetch_grpc_plugin_binary()
\ No newline at end of file
+fetch_grpc_plugin(version = '1.3.0')
\ No newline at end of file