lib.filesystem.listFilesRecursive: only flatten once (#449744)

    ```

  */

  listFilesRecursive =

    let

      # We only flatten at the very end, as flatten is recursive.

      internalFunc =

        dir:

    lib.flatten (

      lib.mapAttrsToList (

        name: type:

        if type == "directory" then

          lib.filesystem.listFilesRecursive (dir + "/${name}")

        else

          dir + "/${name}"

      ) (builtins.readDir dir)

    );

        (lib.mapAttrsToList (

          name: type: if type == "directory" then internalFunc (dir + "/${name}") else dir + "/${name}"

        ) (builtins.readDir dir));

    in

    dir: lib.flatten (internalFunc dir);

  /**

    Transform a directory tree containing package files suitable for