cmd/go/internal/modget: print a fatal error if -d=false
Between Go 1.18 and Go 1.22 go get printed a fatal error if -d was
explicitly set to false. That behavior was reverted in CL 572176, when
we made the -d flag a no-op, but it would make it easier to remove the
-d flag in the future if we continue to print a fatal error if -d is
explicitly set to false.
This change brings back the fatal error for -d=false while keeping the
warning printed for -d=true.
For #43684
Change-Id: I38ae3a3619d408c0237ff485ddee4403b8188abd
Reviewed-on: https://go-review.googlesource.com/c/go/+/591135
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Sam Thanawalla <samthanawalla@google.com>