allow Home Manager to be used as a NixOS module

This is a NixOS module that is intended to be imported into a NixOS
system configuration. It allows the system users to be set up directly
from the system configuration.

The actual profile switch is performed by a oneshot systemd unit per
configured user that acts much like the regular `home-manager switch`
command.

With this implementation, the NixOS module does not work properly with
the `nixos-rebuild build-vm` command. This can be solved by using the
`users.users.<name?>.packages` option to install packages but this
does not work flawlessly with certain Nixpkgs packages. In particular,
for programs using the Qt libraries.

modules/programs/home-manager.nix

@@ -32,7 +32,7 @@ in
     };
   };
 
-  config = mkIf cfg.enable {
+  config = mkIf (cfg.enable && !config.nixosSubmodule) {
     home.packages = [
       (import ../../home-manager {
         inherit pkgs;