PoC working SP module system

+ simple-nixos-mailserver as an SP module

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

@@ -0,0 +1,33 @@
+{
+  description = "PoC SP module for the simple-nixos-mailserver";
+
+  inputs.mailserver.url =
+    gitlab:simple-nixos-mailserver/nixos-mailserver;
+
+  outputs = { self, mailserver }: {
+    # tricks to rename (alias) the original module
+    nixosModules.default = args@{ pkgs, ... }:
+      let
+        module = mailserver.nixosModules.default args;
+      in
+      module // {
+        imports = module.imports ++ [
+          ./config.nix
+          ({ config, ... }: {
+            mailserver =
+              config.selfprivacy.userdata.simple-nixos-mailserver;
+          })
+        ];
+        options = module.options // {
+          selfprivacy.userdata.simple-nixos-mailserver =
+            module.options.mailserver;
+        };
+      };
+    configPathsNeeded =
+      builtins.fromJSON (builtins.readFile ./config-paths-needed.json);
+
+    # TODO generate json docs from module? something like:
+    # nix eval --impure --expr 'let flake = builtins.getFlake (builtins.toPath ./.); pkgs = flake.inputs.mailserver.inputs.nixpkgs.legacyPackages.x86_64-linux; in (pkgs.nixosOptionsDoc { inherit (pkgs.lib.evalModules { modules = [ flake.nixosModules.default ]; }) options; }).optionsJSON'
+    # (doesn't work because of `assertions`)
+  };
+}