cmd/go: fix vet

Now that vet does typechecking, it should use only pkg.gofiles, instead
of pkg.allgofiles. Ignored files should not be checked by vet, because
they wouldn't typecheck.

Fixes issue 4906.

R=rsc, r
CC=golang-dev
https://codereview.appspot.com/7401051

Committer: Rob Pike <r@golang.org>