[release-branch.go1.16] archive/zip: only return directory once via io/fs.FS

While we're here fix the ModTime value for directories.

For #43872
For #45345
Fixes #45347

Change-Id: I155e6517713ef6a9482b9431f1167a44337c6ad2
Reviewed-on: https://go-review.googlesource.com/c/go/+/311530


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
(cherry picked from commit 87e4dcd4)
Reviewed-on: https://go-review.googlesource.com/c/go/+/315249
Trust: Jeremy Faller <jeremy@golang.org>