The fs.* files are already part of the libbitcoin_util library. With the
introduction of the fs_helpers.* it makes sense to move fs.* into the
util/ directory as well.
This concludes backport of core#27254
https://github.com/bitcoin/bitcoin/pull/27254/commits/00e9b97f37e0bdf4c647236838c10b68b7ad5be3