[release-branch.go1.10] cmd/go: run vet on packages with only cgo files

CgoFiles is not included in GoFiles, so we need to check both.

Fixes #24193

Change-Id: I6a67bd912e3d9a4be0eae8fa8db6fa8a07fb5df3
Reviewed-on: https://go-review.googlesource.com/99175


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/102785


Run-TryBot: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>