cmd/go: fix vet

The IgnoredGoFiles are already listed in allgofiles,
so they were being run twice. Worse, the ones in
IgnoredGoFiles are not fully qualified paths, so they
weren't being found when executed outside the
package directory.

Fixes #4764.

R=golang-dev, minux.ma, franciscossouza
CC=golang-dev
https://golang.org/cl/7308049

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(stringList(pkg.allgofiles, pkg.IgnoredGoFiles)))
+		run(tool("vet"), relPaths(pkg.allgofiles))
 	}
 }