retroarch: refactor (#358405)

        services.xserver.enable = true;

        services.xserver.desktopManager.retroarch = {

          enable = true;

          package = pkgs.retroarchBare;

          package = pkgs.retroarch-bare;

        };

        services.xserver.displayManager = {

          sddm.enable = true;

The basic steps to add a new core are:

1. Add a new core using `mkLibretroCore` function (use one of the existing

   files as an example)

   cores as an example)

2. Add your new core to [`default.nix`](./default.nix) file

3. Try to build your core with `nix-build -A libretro.<core>`

## Using RetroArch with cores

To create a custom RetroArch derivation with the cores you want (instead of

using `retroarchFull` that includes all cores), you can use `.override` like

using `retroarch-full` that includes all cores), you can use `.withCores` like

this:

```nix

{ pkgs, ... }:

let

  retroarchWithCores = (pkgs.retroarch.override {

    cores = with pkgs.libretro; [

  retroarchWithCores = (pkgs.retroarch.withCores (cores: with cores; [

    bsnes

    mgba

    quicknes

    ];

  });

  ]));

in

{

  environment.systemPackages = [

  ];

}

```

For advanced customization, see `wrapRetroArch` wrapper.