go/build: no back slash in FindTree returned pkg name
go/build: no back slash in FindTree returned pkg name

Fixes issue 2652.

R=adg, rsc
CC=golang-dev
http://codereview.appspot.com/5516045

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