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

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go