cmd/go/internal/modget: warn -d is a no-op
Updates #43684
Change-Id: I4aa340b1b819374159b5b6b99c14e3f70bfa1bb0
GitHub-Last-Rev:
c380648b9aeeb6c9fd4f0a42ada7921ba99f45bb
GitHub-Pull-Request: golang/go#66362
Reviewed-on: https://go-review.googlesource.com/c/go/+/572176
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>