ListModules was used to download .info files so that 'go list -m all'
would succeed later when offline. However, 'go list -m all' may
already fail when offline after 'go mod tidy', so it doesn't make
sense to add complexity to 'go get'.
Instead, remove the ListModules call and fix the test that
accidentally depended on it.
For #42723
Change-Id: I692597cf5ca15c23fa6fc9d2bac4b6e044299482
Reviewed-on: https://go-review.googlesource.com/c/go/+/271577
Trust: Jay Conrod <jayconrod@google.com> Reviewed-by: Bryan C. Mills <bcmills@google.com>