Fix `foldUserSysteml` anti-pattern

[eureka-cpu]

NixOS modules have an attribute set with imports, config and options as defined here: https://nixos.wiki/wiki/NixOS_modules#Structure and flake outputs have nixosModules and nixosConfigurations attributes which should be followed. Currently foldUserSysteml and the modules of the file tree don't follow this pattern directly. This might be okay for single systems, but when interacting between systems in a configuration they should have some uniformity.

Assuming each user in the tree will have a flake for each system, with its own modules and configurations.