And use it in two internal windows packages, so they don't show up in
"go list std" or binary releases on non-Windows platforms.

Fixes #31920

Change-Id: Iaa292b6015c9d7310dd677c9e296006440ba5e27
Reviewed-on: https://go-review.googlesource.com/c/go/+/175983Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>