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

hosts/nixos/x86_64-linux/__template__/configuration.nix

@@ -1,11 +1,11 @@
 # This is your system's configuration file.
 # Use this to configure your system environment (it replaces /etc/nixos/configuration.nix)
 
-{ inputs, outputs, lib, pkgs, config, ... }: {
+{ inputs, lib, pkgs, config, ... }: {
   # You can import other NixOS modules here
   imports = [
     # If you want to use modules your own flake exports (from modules/nixos):
-    # outputs.nixosModules.example
+    # inputs.self.nixosModules.example
 
     # Or modules from other flakes (such as nixos-hardware):
     # inputs.hardware.nixosModules.common-cpu-amd
@@ -22,8 +22,8 @@
     # You can add overlays here
     overlays = [
       # If you want to use overlays your own flake exports (from overlays dir):
-      # outputs.overlays.modifications
-      # outputs.overlays.additions
+      # inputs.self.overlays.modifications
+      # inputs.self.overlays.additions
 
       # Or overlays exported from other flakes:
       # neovim-nightly-overlay.overlays.default