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

pkgs/default.nix

@@ -1,23 +0,0 @@
-# Custom packages, that can be defined similarly to ones from nixpkgs
-# You can build them using 'nix build .#example' or (legacy) 'nix-build -A example'
-
-{ pkgs ? (import ../nixpkgs.nix) { }
-, ...
-}: {
-  # example = pkgs.callPackage ./example { };
-  advcpmv = pkgs.callPackage ./advcpmv { };
-  circom = pkgs.callPackage ./circom { };
-  circom-lsp = pkgs.callPackage ./circom-lsp { };
-  envsub = pkgs.callPackage ./envsub { };
-  fennel-language-server = pkgs.callPackage ./fennel-language-server { };
-  flamelens = pkgs.callPackage ./flamelens { };
-  parinfer-rust = pkgs.callPackage ./parinfer-rust { };
-  pest-ide-tools = pkgs.callPackage ./pest-ide-tools { };
-  # FIXME: only buildable on darwin
-  # pngpaste = pkgs.callPackage ./pngpaste { };
-  srtool-cli = pkgs.callPackage ./srtool-cli { };
-  # FIXME: does not build
-  # swww = pkgs.callPackage ./swww { };
-  vim-fmi-cli = pkgs.callPackage ./vim-fmi-cli { };
-  win2xcur = pkgs.callPackage ./win2xcur { };
-}

pkgs/pngpaste/systems.nix

@@ -0,0 +1,2 @@
+{ lib, hostPlatform, targetPlatform, ... }:
+targetPlatform.isDarwin

pkgs/swww/systems.nix

@@ -0,0 +1,3 @@
+{ lib, hostPlatform, targetPlatform, ... }:
+# BUG: does not build
+false