cmd/go: prevent packages from being cleaned more than once
cmd/go: prevent packages from being cleaned more than once

If a package was listed as a dependency from multiple places, it
could have been cleaned repeatedly.

R=golang-dev, dave, rsc, seed, bradfitz
CC=golang-dev, minux.ma
https://codereview.appspot.com/7482043

Committer: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go