cmd/go/internal/modfetch: re-resolve commit hashes in readDiskStat

Previously, when we resolved a commit hash (not a complete version),
we always checked the contents of the module cache for any
pseudo-version matching that commit.

However, there are many possible names for a given commit. Generally
the semantically-highest valid name is the best, and that may change
over time as new tags are added, so if we are able to fetch a better
name from upstream we should do so. Otherwise, we should fall back to
the highest appropriate name found in the cache.

Fixes #27171
Updates #27173

Change-Id: Ib5c7d99eb463af84674e969813039cbbee7e395b
Reviewed-on: https://go-review.googlesource.com/c/go/+/182178


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