lib/filesystem: remove absolute path literals (#514225)

        in

        if builtins.length matches != 0 then

          { inherit path matches; }

        else if path == /. then

        else if toString path == "/" then

          null

        else

          go (dirOf path);

          base = baseNameOf file;

          type = (builtins.readDir parent).${base} or null;

        in

        file == /. || type == "directory";

        toString file == "/" || type == "directory";

    in

    go (if isDir then file else parent);