cmd/go/internal/modload: query correct "latest" version through proxy

This fixes a regression introduced in CL 180337. When we query a
module at "latest" that has no tagged versions, we tried to use ""
as the version because we used info.Name instead of info.Version. This
only happened when using a proxy: in direct mode, info.Name is set to
the underlying VCS revision, which is fine.

Also: serve "/mod/path/@latest" through our test proxy.
Previously, we served a 404, which made this bug hard to detect.

Fixes #32636

Change-Id: I5c60975656297f862cad66675170e819685ebd39
Reviewed-on: https://go-review.googlesource.com/c/go/+/182697


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