fix config attributes contrain mechanism for SP modules

Now it should work for all nested imports too. `imports` are traversed
recursively to redefine each imported module function with altered one,
constraining its config attribute (respecting config-paths-needed.json).

sp-modules/simple-nixos-mailserver/flake.nix

@@ -6,17 +6,14 @@
 
   outputs = { self, mailserver }: {
     # tricks to rename (alias) the original module
-    nixosModules.default = args@{ pkgs, ... }:
+    nixosModules.default = args@{ pkgs, config, ... }:
       let
         module = mailserver.nixosModules.default args;
       in
       module // {
         imports = module.imports ++ [
           ./config.nix
-          ({ config, ... }: {
-            mailserver =
-              config.selfprivacy.userdata.simple-nixos-mailserver;
-          })
+          { mailserver = config.selfprivacy.userdata.simple-nixos-mailserver; }
         ];
         options = module.options // {
           selfprivacy.userdata.simple-nixos-mailserver =