cmd/go/internal/modfetch: simplify handling of weird version tags

cmd/go/internal/modfetch: simplify handling of weird version tags

This fixes an obscure bug in 'go list -versions' if the repo contains
a tag with an explicit "+incompatible" suffix. However, I've never
seen such a repo in the wild; mostly it's an attempt to wrap my brain
around the code and simplify things a bit for the future.

Updates #51324
Updates #51312

Change-Id: I1b078b5db36470cf61aaa85b5244c99b5ee2c842
Reviewed-on: https://go-review.googlesource.com/c/go/+/387917
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>