This assumes #158594 is implemented as an incremental change to the module system. I'm writing this as a separate issue to manage the scope; feel free to ignore this one.

For modules with large amounts of unset options, it is inefficient for module system evaluation to be guided by the opts, as the defs are significantly smaller value. In its early history, module system evaluation primarily traversed config, but presumably, this was abandoned with the introduction of defaults in options.

Making this mode of module evaluation available on an opt-in basis would allow large, frequently used submodules such as the ones in systemd to be modeled more efficiently, providing documentation, type checking and merging for this important part of NixOS.

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