These functions only work with the filesystem, they don't import
anything as sources

Co-Authored-By: Robert Hensing <robert@roberthensing.nl>

Co-Authored-By: Robert Hensing <robert@roberthensing.nl>

Previously this function couldn't handle / being passed, it would throw
an error:

error: attribute '' missing

       at nixpkgs/lib/filesystem.nix:24:20:

           23|   */
           24|   pathType = path: (readDir (dirOf path)).${baseNameOf path};
             |                    ^
           25|

Consequently this also fixes the
lib.filesystem.{pathIsDirectory,pathIsRegularFile} functions.

Previously it would fail with

  error: attribute 'nonexistent' missing

         at nixpkgs/lib/filesystem.nix:29:10:

             28|     if dirOf path == path then "directory"
             29|     else (readDir (dirOf path)).${baseNameOf path};
               |          ^
             30|

Co-Authored-By: Robert Hensing <robert@roberthensing.nl>

Co-Authored-By: Robert Hensing <robert@roberthensing.nl>