Move all of NixOS to nixos/ in preparation of the repository merge

nixos/default.nix

@@ -0,0 +1,44 @@
+{ configuration ? import ./lib/from-env.nix "NIXOS_CONFIG" <nixos-config>
+, system ? builtins.currentSystem
+, nixpkgs ? <nixpkgs>
+}:
+
+let
+
+  eval = import ./lib/eval-config.nix {
+    inherit system nixpkgs;
+    modules = [ configuration ];
+  };
+
+  inherit (eval) config pkgs;
+
+  # This is for `nixos-rebuild build-vm'.
+  vmConfig = (import ./lib/eval-config.nix {
+    inherit system nixpkgs;
+    modules = [ configuration ./modules/virtualisation/qemu-vm.nix ];
+  }).config;
+
+  # This is for `nixos-rebuild build-vm-with-bootloader'.
+  vmWithBootLoaderConfig = (import ./lib/eval-config.nix {
+    inherit system nixpkgs;
+    modules =
+      [ configuration
+        ./modules/virtualisation/qemu-vm.nix
+        { virtualisation.useBootLoader = true; }
+      ];
+  }).config;
+
+in
+
+{
+  inherit eval config;
+
+  system = config.system.build.toplevel;
+
+  vm = vmConfig.system.build.vm;
+
+  vmWithBootLoader = vmWithBootLoaderConfig.system.build.vm;
+
+  # The following are used by nixos-rebuild.
+  nixFallback = pkgs.nixUnstable;
+}