[release-branch.go1.17] cmd/go: ensure 'go get -u' can upgrade pruned (1.17+) modules

Fixes #47857

Change-Id: I981a31ba4ff716570ac4c6f35b289fa480faa5bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/343879


Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit bdc1bef8)
Reviewed-on: https://go-review.googlesource.com/c/go/+/346211