cmd/go: skip _obj directories in package scans
cmd/go: skip _obj directories in package scans

Fixes issue 2693

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5557057
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go