Refactor back to a more modular configuration

modules/nixos/core/openssh.nix

@@ -0,0 +1,23 @@
+{ config, lib, ... }:
+
+with lib;
+
+let
+  cfg = config.nixosModules.core.openssh;
+in
+{
+  options.nixosModules.core.openssh.enable = mkEnableOption "Enable openssh";
+
+  config = mkIf cfg.enable {
+    services.openssh = {
+      enable = true;
+
+      openFirewall = lib.mkForce true;
+
+      settings = {
+        PasswordAuthentication = false;
+        PermitRootLogin = lib.mkForce "no";
+      };
+    };
+  };
+}