cmd/go: explicitly reject 'list -u' and 'list -versions' when '-mod=vendor' is set

The information requested by these flags is not available from the
vendor directory.

Noticed while diagnosing #36478.

Updates #33848

Change-Id: I2b181ba5c27f01fdd6277d8d0ab1003c05774ff7
Reviewed-on: https://go-review.googlesource.com/c/go/+/214081


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>