gofmt: don't attempt certain illegal rewrites
gofmt: don't attempt certain illegal rewrites

(e.g.: echo 'package main' | gofmt -r 'x->7'
cannot change the package name to 7)

R=rsc
CC=golang-dev
http://codereview.appspot.com/3913041

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