goprotobuf: make the generator a package, with the main
program a separate file.  also introduce the hooks for a plugin
(really a plugin for this plugin) to enable add-on generators for
things like RPC.

R=rsc
CC=golang-dev
http://codereview.appspot.com/970046
diff --git a/compiler/descriptor/Makefile b/compiler/descriptor/Makefile
index cd2add9..5c7d716 100644
--- a/compiler/descriptor/Makefile
+++ b/compiler/descriptor/Makefile
@@ -35,6 +35,8 @@
 GOFILES=\
 	descriptor.pb.go\
 
+DEPS=../../proto
+
 include $(GOROOT)/src/Make.pkg
 
 # Not stored here, but descriptor.proto is in http://code.google.com/p/protobuf