[WIP] Tighten file-api dataclasses

[LecrisUT]

trafficstars

This one also tries to load the Directory object, but it looses the original data from the CodeModel's field. Ideally this should be revisited so that it can inherit the fields if available. ~~My thought is to have them subclassed and pass along all the parent fields. Or more robustly, we should split them into the individual versioned objects, wdyt?~~

For debugging purposes I've made the file-api failure an error, but should we make that permanent in the future?

[LecrisUT]

I guess this is ready for review. I tried to consider versioned dataclasses, but the lack of kw_only is blocking it. I have tried to backport it, but no luck on either the logic or mypy.