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/Makefile b/compiler/Makefile
index ad85973..8da2086 100644
--- a/compiler/Makefile
+++ b/compiler/Makefile
@@ -36,7 +36,7 @@
 GOFILES=\
 	main.go\
 
-DEPS=descriptor plugin
+DEPS=generator descriptor plugin
 
 include $(GOROOT)/src/Make.cmd