nixos: use functionTo to prevent evaluation errors while merging

Without this patch merging options like
services.xserver.windowManager.xmonad.extraPackages
results in the evaluation error:

  error: value is a list while a set was expected, at nixpkgs/lib/options.nix:77:23

With this patch we get the desired merging behaviour that just concatenates the
resulting package lists.

(cherry picked from commit 6e99f9fdecb1f28308c8e0aed0fc851737354864)

Co-Authored-By: Silvan Mosberger <contact@infinisil.com>

nixos/modules/services/development/hoogle.nix

@@ -25,6 +25,7 @@ in {
     };
 
     packages = mkOption {
+      type = types.functionTo (types.listOf types.package);
       default = hp: [];
       defaultText = "hp: []";
       example = "hp: with hp; [ text lens ]";

nixos/modules/services/misc/gitit.nix

@@ -42,6 +42,7 @@ let
       };
 
       extraPackages = mkOption {
+        type = types.functionTo (types.listOf types.package);
         default = self: [];
         example = literalExample ''
           haskellPackages: [

nixos/modules/services/misc/ihaskell.nix

@@ -21,6 +21,7 @@ in
       };
 
       extraPackages = mkOption {
+        type = types.functionTo (types.listOf types.package);
         default = self: [];
         example = literalExample ''
           haskellPackages: [

nixos/modules/services/misc/octoprint.nix

@@ -66,6 +66,7 @@ in
       };
 
       plugins = mkOption {
+        type = types.functionTo (types.listOf types.package);
         default = plugins: [];
         defaultText = "plugins: []";
         example = literalExample "plugins: with plugins; [ themeify stlviewer ]";

nixos/modules/services/monitoring/netdata.nix

@@ -77,6 +77,7 @@ in {
           '';
         };
         extraPackages = mkOption {
+          type = types.functionTo (types.listOf types.package);
           default = ps: [];
           defaultText = "ps: []";
           example = literalExample ''

nixos/modules/services/networking/supybot.nix

@@ -64,6 +64,7 @@ in
       };
 
       extraPackages = mkOption {
+        type = types.functionTo (types.listOf types.package);
         default = p: [];
         description = ''
           Extra Python packages available to supybot plugins. The

nixos/modules/services/x11/window-managers/exwm.nix

@@ -37,6 +37,7 @@ in
         description = "Enable an uncustomised exwm configuration.";
       };
       extraPackages = mkOption {
+        type = types.functionTo (types.listOf types.package);
         default = self: [];
         example = literalExample ''
           epkgs: [

nixos/modules/services/x11/window-managers/xmonad.nix

@@ -53,6 +53,7 @@ in {
       };
 
       extraPackages = mkOption {
+        type = types.functionTo (types.listOf types.package);
         default = self: [];
         defaultText = "self: []";
         example = literalExample ''