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 #4906.

R=rsc, r
CC=golang-dev
https://golang.org/cl/7401051

src/cmd/go/vet.go

@@ -32,6 +32,6 @@ func runVet(cmd *Command, args []string) {
 		// Use pkg.gofiles instead of pkg.Dir so that
 		// the command only applies to this package,
 		// not to packages in subdirectories.
-		run(tool("vet"), relPaths(pkg.allgofiles))
+		run(tool("vet"), relPaths(pkg.gofiles))
 	}
 }