Implements a read-only fs.FS view of a txtar.Archive.
It returns an error if the names in the archive are not valid
file system names. The archive cannot be modified while the
file system is in use.
Fixesgolang/go#44158
Change-Id: If1e77833545a5b5db28006322e7f214951bc52f6
Reviewed-on: https://go-review.googlesource.com/c/tools/+/598756
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Tim King