os: use filepathlite.IsAbs

os: use filepathlite.IsAbs

It is better to have a single implementation of IsAbs, which is quite
tricky to get right on Windows.

Change-Id: I45933b0ceff2920d9eddb61e62aacb2602c3dc8c
Reviewed-on: https://go-review.googlesource.com/c/go/+/582498
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>