The NixOS home-manager module only uses nixpkgs settings if
useGlobalPkgs is false. If it is true, there is no need to propagate
config (and propagating nixpkgs.config would cause an assertion).
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.
The `propagationModule` config option provides a module that can be used
to propagate flakelight configuration into other module systems such as
NixOS or home-manager.
