nix: Use naming convention for hardware-configuration

Although I could just integrate this directly in configuration.nix since
everything is a module, having a separate hardware-configuration.nix
makes it easier to integrate with other devices that may output
different configurations.

host/default.nix

@@ -1,8 +0,0 @@
-{
-  imports = [
-    ./laptop
-    ./mullvad
-    ./networking
-    ./virtualization
-  ];
-}