Merge pull request #267324 from tweag/lib.fileset-minor

## To update in the future

Here's a list of places in the library that need to be updated in the future:

- > The file set library is currently somewhat limited but is being expanded to include more functions over time.

  in [the manual](../../doc/functions/fileset.section.md)

- If/Once a function exists that can optionally include a path depending on whether it exists, the error message for the path not existing in `_coerce` should mention the new function

            - Set `root` to ${toString fileset._internalBase} or any directory higher up. This changes the layout of the resulting store path.

            - Set `fileset` to a file set that cannot contain files outside the `root` (${toString root}). This could change the files included in the result.''

    else

      builtins.seq sourceFilter

      seq sourceFilter

      cleanSourceWith {

        name = "source";

        src = root;