nixos-config/overlays/file-tree.nix

_: prev: let
  inherit (prev) lib;
  linkToPath = path: fileOrDir: (
    if prev.lib.types.path.check fileOrDir
    then ["ln -sT ${fileOrDir} ${path}"]
    else
      ["mkdir -p ${path}"]
      ++ lib.concatLists (
        lib.mapAttrsToList
        (dirName: linkToPath "${path}/${dirName}")
        fileOrDir
      )
  );
in {
  setToDirectories = files:
    prev.runCommand "set-to-directories" {}
    (lib.concatStringsSep "\n" (linkToPath "$out" files));
}