cmd/go/internal/modload: do not disable Query for -mod=readonly

'go list -m' allows explicit module@version arguments,
which it resolves (using Query) but does not add to the build list.
Similarly, 'go list -u' resolves versions without modifying the build list.

These explicit operations should be allowed even when '-mod=readonly' is set.

Updates #36478

'go list' and 'go mod download' do not

Change-Id: I5d2735729ad573635b9c1902d5d3a8bd960b8a76
Reviewed-on: https://go-review.googlesource.com/c/go/+/214077


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