cmd/go/internal/modfetch: use the resolved version to search for tags in (*codeRepo).convert

Previously, we used the passed-in statVers as the basis for tag search,
but it is not always valid.
Instead, use info.Name, which (by precondition) must be valid.

Updates #32161
Updates #27171

Change-Id: Iaecb5043bdf2fefd26fbe3f8e3714b07d22f580f
Reviewed-on: https://go-review.googlesource.com/c/go/+/179857


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