cmd/go: allow -mod=vendor to be set outside of a module

cmd/go: allow -mod=vendor to be set outside of a module

It will behave as if teh command-line-arguments module was vendored,
with zero dependencies, much as -mod=readonly works.

Fixes #56536

Change-Id: Ia02dda2f90d006b5917c3ae002ccb714987f27a3
Reviewed-on: https://go-review.googlesource.com/c/go/+/448019
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Michael Matloob <matloob@golang.org>