nixos: create 'modules/nixos' folder

Let's consolidate all modules under one path, so that NixOS,
home-manager, and nix-darwin (if I ever end up using it down the line)
would go under the same folder.

modules/nixos/hardware/networking/default.nix

@@ -0,0 +1,27 @@
+{ config, lib, ... }:
+let
+  cfg = config.my.hardware.networking;
+in
+{
+  options.my.hardware.networking = with lib; {
+    externalInterface = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      example = "eth0";
+      description = ''
+        Name of the network interface that egresses to the internet. Used for
+        e.g. NATing internal networks.
+      '';
+    };
+
+    wireless = {
+      enable = mkEnableOption "wireless configuration";
+    };
+  };
+
+  config = lib.mkMerge [
+    (lib.mkIf cfg.wireless.enable {
+      networking.networkmanager.enable = true;
+    })
+  ];
+}