refactor: Organized modules like they are in NixOS/nixpkgs.

nixos/modules/virtualisation/vagrant.nix

@@ -0,0 +1,43 @@
+{ config, ... }: {
+  config = {
+    nix.settings.experimental-features = [ "nix-command" "flakes" ];
+
+    virtualisation.virtualbox.guest.enable = true;
+
+    # Mount a VirtualBox shared folder.
+    fileSystems."/vagrant" = {
+      fsType = "vboxsf";
+      device = "vagrant";
+      options = [ "rw,uid=1001,gid=60,_netdev" ]; # mount as vagrant:nginx
+    };
+
+    networking.extraHosts = ''
+      127.0.0.1 ${config.networking.hostName}.local
+    '';
+
+    security.sudo.wheelNeedsPassword = false;
+
+    users.users.root.password = "vagrant";
+    users.users."vagrant" = {
+      isNormalUser = true;
+      password = "vagrant";
+      extraGroups = [
+        "nginx"
+        "wheel"
+        # Allow mounting of shared folders.
+        "vboxsf"
+      ];
+    };
+
+    services.openssh.enable = true;
+
+    services.avahi = {
+      enable = true;
+      publish = {
+        enable = true;
+        addresses = true;
+        workstation = false;
+      };
+    };
+  };
+}