cmd/go/internal/vet: skip -fix on pkgs from vendor or non-main mod

cmd/go/internal/vet: skip -fix on pkgs from vendor or non-main mod

This change causes go fix (and go vet -fix) to skip applying fixes
to any package in the vendor/ tree, including the GOROOT vendor
packages that are part of std, and to any package from a non-main
module (since these usually come from the readonly module cache).

+ test

Fixes golang/go#76479

Change-Id: Ifdb73e09fbe413b4d99a92e5081b8ea43460be0b
Reviewed-on: https://go-review.googlesource.com/c/go/+/727300
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>