Move error types and filesystem-specific functions to a separate file,
and keep the fs:: namespace in public exports.
Change-Id: I5e9e83ad78d9aea38553fafc293d3e4f8c31a8c1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11486
Tested-by: BuildkiteCI
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Autosubmit: flokli <flokli@flokli.de>
