go/build: add check for allowed vendor packages

go/build: add check for allowed vendor packages

Add a test that checks that all vendored packages in both src/ and
src/cmd are on an allowlist.

For #65678.

Change-Id: Ie9982f8b69c9c0fc6fef41c9dadeb464ecd7781a
Reviewed-on: https://go-review.googlesource.com/c/go/+/564355
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>