cmd/go/internal/gover: add new Go version package

cmd/go/internal/gover: add new Go version package

Clean up Go version comparison.

CL 494436 added an ad hoc version comparison for the toolchain switch.

There are also other version comparisons scattered throughout the code,
assuming that using semver.Compare with a "v" prefix gives the right answer.
As we start to allow versions like "go 1.21rc1" in the go.mod file,
those comparisons will not work properly.

A future CL will need to inject Go versions into semver for use with MVS,
so do what Bryan suggested in the review of CL 494436 and rewrite the
comparison in terms of that conversion.

For #57001.

Change-Id: Ia1d441f1bc259874c6c1b3b9349bdf9823a707d4
Reviewed-on: https://go-review.googlesource.com/c/go/+/496735
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>