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

tv42/old-go,pombredanne/go-deleteme,tv42/old-go,ssrl/go,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,ssrl/go,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,ssrl/go,dsriram/golang,tv42/old-go,ssrl/go,apparentlymart/golang,dsriram/golang,ssrl/go,dsriram/golang,gnanderson/go,apparentlymart/golang,tv42/old-go,apparentlymart/golang,ssrl/go,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,ssrl/go,tv42/old-go,dsriram/golang,tv42/old-go,apparentlymart/golang,apparentlymart/golang