internal/goversion: add new package, move Go 1.x constant there out of go/build

Found by Josh, who says in the bug that it shrinks cmd/compile by 1.6 MB (6.5%).

Fixes #31563

Change-Id: I35127af539630e628a0a4f2273af519093536c38
Reviewed-on: https://go-review.googlesource.com/c/go/+/172997


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>