Move Nix files into a nix directory

Move everything from `config` to the root level.

nix/lib/nixos/default.nix

@@ -0,0 +1,64 @@
+{
+  inputs,
+  outputs,
+  nixos-hardware,
+  pkgs,
+  self,
+  username,
+}:
+{
+  desktop ? false,
+  hostname,
+}:
+let
+  configuration = import ./configuration.nix {
+    inherit
+      outputs
+      desktop
+      hostname
+      inputs
+      self
+      ;
+  };
+  hardwareConfiguration = import ./hardware-configuration.nix;
+in
+inputs.nixpkgs.lib.nixosSystem {
+  modules = [
+    inputs.home-manager.nixosModules.home-manager
+    {
+      home-manager = {
+        extraSpecialArgs = {
+          inherit
+            inputs
+            desktop
+            self
+            username
+            ;
+        };
+        useGlobalPkgs = true;
+        useUserPackages = true;
+        users."${username}" = import "${self}/nix/home/${username}";
+      };
+    }
+
+    (import ../../modules/nixos/autorandr.nix)
+    (import ../../modules/nixos/i3.nix {
+      inherit
+        inputs
+        pkgs
+        self
+        username
+        ;
+    })
+
+    configuration
+    hardwareConfiguration
+
+    # TODO: only for "lemp11".
+    nixos-hardware.nixosModules.common-cpu-intel
+    nixos-hardware.nixosModules.common-gpu-intel
+    nixos-hardware.nixosModules.common-pc-laptop
+    nixos-hardware.nixosModules.common-pc-laptop-hdd
+    nixos-hardware.nixosModules.system76
+  ];
+}