nixos/users: Populate group members option

This change makes it so that accessing config.users.groups.*.members isn't
empty by default, but instead contains all the users whose `extraGroups`
includes that group, allowing fancy things like

  { config, ... }: {
    users.groups.libvirt.members = config.users.groups.wheel.members;
  }

to add all users in the wheel group to the libvirt group

nixos/modules/config/users-groups.nix

@@ -324,7 +324,7 @@ let
 
   };
 
-  groupOpts = { name, ... }: {
+  groupOpts = { name, config, ... }: {
 
     options = {
 
@@ -358,6 +358,10 @@ let
 
     config = {
       name = mkDefault name;
+
+      members = mapAttrsToList (n: u: u.name) (
+        filterAttrs (n: u: elem config.name u.extraGroups) cfg.users
+      );
     };
 
   };
@@ -419,12 +423,7 @@ let
           initialPassword initialHashedPassword;
         shell = utils.toShellPath u.shell;
       }) cfg.users;
-    groups = mapAttrsToList (n: g:
+    groups = attrValues cfg.groups;
-      { inherit (g) name gid;
-        members = g.members ++ (mapAttrsToList (n: u: u.name) (
-          filterAttrs (n: u: elem g.name u.extraGroups) cfg.users
-        ));
-      }) cfg.groups;
   });
 
   systemShells =