Update to (somewhat?) dendritic pattern

Heavily inspired by https://github.com/Doc-Steve/dendritic-design-with-flake-parts.

> Atomic commits? Never heard of them!

modules/nix/flake-parts/lib.nix

@@ -0,0 +1,33 @@
+{ inputs
+, lib
+, ...
+}: {
+  # Helper functions for creating system / home-manager configurations
+
+  options.flake.lib = lib.mkOption {
+    type = lib.types.attrsOf lib.types.unspecified;
+    default = { };
+  };
+
+  config.flake.lib = {
+    mkNixos = system: name: {
+      ${name} = inputs.nixpkgs.lib.nixosSystem {
+        modules = [
+          inputs.self.modules.nixos.${name}
+          { nixpkgs.hostPlatform = lib.mkDefault system; }
+          { nixpkgs.config.allowUnfree = true; }
+        ];
+      };
+    };
+
+    mkHomeManager = system: name: {
+      ${name} = inputs.home-manager.lib.homeManagerConfiguration {
+        pkgs = inputs.nixpkgs.legacyPackages.${system};
+        modules = [
+          inputs.self.modules.homeManager.${name}
+          { nixpkgs.config.allowUnfree = true; }
+        ];
+      };
+    };
+  };
+}