The type we use to expose write operations is still evolving. It might
be a single writefs.FS interface, or similar to go where we have an
interface per feature (e.g. writefs.MkdirFS). These choices are all
implementation details for DirFS and won't be settled before the end of
the month version cutoff. Instead, this only exposes the ability to
create a DirFS, not an arbitrary implementation of writefs.FS. This does
so by making `writefs.FS` an internal type.
Signed-off-by: Adrian Cole <adrian@tetrate.io>
07a814a105