goprotobuf: Correctly generate extension names for .proto files that don't declare a package.

R=r
CC=golang-dev
http://codereview.appspot.com/5653063
diff --git a/proto/extensions.go b/proto/extensions.go
index 9f840fa..70297c2 100644
--- a/proto/extensions.go
+++ b/proto/extensions.go
@@ -62,7 +62,7 @@
 	ExtendedType  interface{} // nil pointer to the type that is being extended
 	ExtensionType interface{} // nil pointer to the extension type
 	Field         int32       // field number
-	Name          string      // fully-qualified name of extension
+	Name          string      // fully-qualified name of extension, for text formatting
 	Tag           string      // protobuf tag style
 }