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://golang.org/cl/7482043

src/cmd/go/clean.go

@@ -106,6 +106,8 @@ func clean(p *Package) {
 	if cleaned[p] {
 		return
 	}
+	cleaned[p] = true
+
 	if p.Dir == "" {
 		errorf("can't load package: %v", p.Error)
 		return