clan template: reduce autogenerate code to a minimum

lib/build-clan/default.nix

@@ -0,0 +1,28 @@
+{ nixpkgs, clan, lib }:
+{ directory  # The directory containing the machines subdirectory
+, specialArgs ? { } # Extra arguments to pass to nixosSystem i.e. useful to make self available
+, machines ? { } # allows to include machine-specific modules i.e. machines.${name} = { ... }
+}:
+let
+  machinesDirs =
+    if builtins.pathExists (directory + /machines)
+    then builtins.readDir (directory + /machines)
+    else { };
+
+  machineSettings = machineName:
+    if builtins.pathExists (directory + /machines/${machineName}/settings.json)
+    then builtins.fromJSON (builtins.readFile (directory + /machines/${machineName}/settings.json))
+    else { };
+
+  nixosConfigurations = lib.mapAttrs
+    (name: _mod:
+      nixpkgs.lib.nixosSystem {
+        modules = [
+          (machineSettings name)
+          (machines.${name} or { })
+        ] ++ lib.attrValues clan.clanModules;
+        specialArgs = specialArgs;
+      })
+    machinesDirs;
+in
+nixosConfigurations