[release-branch.go1.16] cmd/go/internal/modfetch: do not short-circuit canonical versions

Since at least CL 121857, the conversion logic in
(*modfetch).codeRepo.Stat has had a short-circuit to use the version
requested by the caller if it successfully resolves and is already
canonical.

However, we should not use that version if it refers to a branch
instead of a tag, because branches (unlike tags) usually do not refer
to a single, stable release: a branch named "v1.0.0" may be for the
development of the v1.0.0 release, or for the development of patches
based on v1.0.0, but only one commit (perhaps at the end of that
branch — but possibly not even written yet!) can be that specific
version.

We already have some logic to prefer tags that are semver-equivalent
to the version requested by the caller. That more general case
suffices for exact equality too — so we can eliminate the
special-case, fixing the bug and (happily!) also somewhat simplifying
the code.

Updates #35671
Fixes #50686
Fixes CVE-2022-23773

Change-Id: I2fd290190b8a99a580deec7e26d15659b58a50b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/378400


Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
(cherry picked from commit fa4d9b8e)
Reviewed-on: https://go-review.googlesource.com/c/go/+/382839