nixos-config/overlays/flake-module.nix

{
  lib,
  inputs,
  ...
}: {
  perSystem = {
    inputs',
    system,
    ...
  }: {
    _module.args.pkgs = inputs'.nixos-stable.legacyPackages.appendOverlays (
      [
        (_: _: {
          flake-inputs = inputs;
          flake-inputs' = inputs';
        })
      ]
      ++ (let
        overlayPath = ./.;
        candidates = lib.attrNames (builtins.readDir overlayPath);
        pathToOverlay = n: overlayPath + ("/" + n);
        isNixFile = n: builtins.match ".*\\.nix" n != null;
        isNixDir = n: builtins.pathExists (pathToOverlay n + "/default.nix");
        notDefault = n: n != "flake-module.nix";
        isOverlay = n: (isNixDir n || isNixFile n) && notDefault n;
        overlays = builtins.filter isOverlay candidates;
        importOverlay = n: import (pathToOverlay n);
      in
        map importOverlay overlays)
    );
  };
}