refactor(flake)!: export packages using a flake module

Also stop passing `outputs` around, prefer `inputs.self`
Also put all `auto_` in `readOnly` `internal` options
- `autoModules`, `autoConfigurations` and `autoPackages`
Do not export packages as overlay (causes an infinite recursion)
- this is documented on the two places it has effect on
-- in `./overlays/default.nix` and `./modules/flake/configurations.nix`
-- in `autoConfigurations` we manually extend `pkgs` with the flake packages
Allow packages to say what `systems` they are compatible with
- See `./pkgs/swww/systems.nix` - disabled for all systems
- See `./pkgs/pngpaste/systems.nix` - enabled only for `darwin` targets

modules/flake/pkgs.nix

@@ -4,6 +4,8 @@
   perSystem = { system, ... }: {
     _module.args.pkgs = import inputs.nixpkgs {
       inherit system;
+      # WARN: not including `self.packages` overylay
+      #       because it causes an infinite recursion
       overlays = lib.attrValues self.overlays ++ [
         inputs.neovim-nightly-overlay.overlays.default
         inputs.zig-overlay.overlays.default