cmd: change from sort functions to slices functions

cmd: change from sort functions to slices functions

Doing this because the slices functions are slightly faster and
slightly easier to use. It also removes one dependency layer.

We did this outside of bootstrap tools in CL 587655.
Now that the bootstrap compiler is 1.22, we can do this in more code.

Change-Id: I9ed2dd473758cacd14f76a0639368523ccdff72f
Reviewed-on: https://go-review.googlesource.com/c/go/+/626038
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>