go/parser: minor cleanup
go/parser: minor cleanup

- there is no label scope at package level
- open/close all scopes symmetrically now
  that there is only one parse entry point
  (parseFile)

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/6230047

firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang