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
Pass around `perSystem`'s `pkgs` instances to the system configurations
Some more refactoring in `configurations`' `mkHost` functions
Also start using `home-manager`'s `useGlobalPkgs` option
Also use `mkShellNoCC` for default `devShell`
Also `nix flake update`
