Enable all options to be auto-loaded from nixDir

This removes the manual configuration of which options are auto-loaded.
Now all options are eligible except for "nixDir" and "_module" as those
would cause inf recursions.

Additionally, instead of setting name aliases in the nixDir module, a
config option is added, enabling other modules to extend the aliases
lists.

builtinModules/propagationModule.nix

@@ -12,7 +12,7 @@ let
   inherit (flakelight.types) module;
 in
 {
-  options.propagationModule = mkOption { type = module; };
+  options.propagationModule = mkOption { type = module; internal = true; };
 
   config.propagationModule =
     { lib, pkgs, options, ... }: