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/factory/flake-option.nix

@@ -0,0 +1,6 @@
+{ lib, ... }: {
+  options.flake.factory = lib.mkOption {
+    type = lib.types.attrsOf lib.types.unspecified;
+    default = { };
+  };
+}