lint: consider requiring that every file in an exercise directory is "used"

[ee7]

Follow-up from https://discord.com/channels/854117591135027261/1085588142163238922/threads/1146023011300937748

Ping @ErikSchierboom.

[ErikSchierboom]

As the Discord discussion mentions, we have several fields that can be used for this:

• files.solution
• files.test
• files.invalidator
• files.editor

I don't think the option of "assume a certain extension for a track" will work in all cases. In fact, any source files should almost invariably be listed in one of files.solution, files.test or files.editor, as those will be files the editor has to show for the student to understand what's happening.

I think the only real option would be to introduce another field, e.g. files.helper or something like that, that could include any other required files. This field would not influence the website, it would just be there for configlet lint to verify its presence.