[release-branch.go1.16] cmd/go: fix version validation in 'go mod edit -exclude'

The fix is to pull in CL 295931 from the x/mod repo.

Updates #44497
Fixes #44498

Change-Id: I008b58d0f4bb48c09d4f1e6ed31d11a714f87dc0
Reviewed-on: https://go-review.googlesource.com/c/go/+/295150


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
(cherry picked from commit 691ac806)
Reviewed-on: https://go-review.googlesource.com/c/go/+/295930