These are various modules ranging from link:https://nixos.org/manual/nixos/stable/index.html#sec-writing-modules[NixOS modules] and link:https://github.com/nix-community/home-manager[home-manager] modules.
This makes the custom modules easier to import whether it's used with flakes or not.
Instead I recommend to make full use of environment-specific module structuring (such as host-specific modules on NixOS, user-specific modules on home-manager) on their respective environment configurations.
As a bonus, this makes it easier to upstream them if we want to.
* That said, custom modules can rely on other custom modules.
Otherwise, we're just limiting ourselves by forcing the modules to be standalone.
Plus we could fix encountered issues with our own solution (and even upstream them if possible).
* Follow the upstream module design as much as possible.
This makes it easier to design custom module extensions around them.
(Also a bonus for easier time upstreaming the module if I want to.)