* nixos: remove with lib
* nix-darwin: remove with lib
* home-manager: remove with lib
* modules/accounts: remove with lib
* modules/config: remove with lib
* modules/i18n: remove with lib
* modules/misc: remove with lib
* modules: remove with lib
* modules/targets: remove with lib
* tests/modules/firefox: remove with lib
* tests/modules/services: remove with lib
Add `preConfig`, which acts like `extraConfig`, but placed before
`settings`. This will allow to overwrite settings in `preConfig`,
using `settings` option.