Start refactoring to model + checks modules (by adding checks for include files)

[gkreitz]

Per #398, we're heading in a direction where we want to split verifyproblem.py into smaller parts. My plan is to add a model module, capturing the data, and a checks module, with all the checks.

Untangling the current pieces gets messy quickly. So, for a first PR properly moving in this direction, I decided to add a "new" thing to the model - include files (which previously completely lacked any checks in verifyproblem), which serves to demonstrate/test the approach and how to integrate it into verifyproblem (without having to do all parts at once).

This PR:

• Removes support for include files having a build file to change compilation (this was never in the specs, and never supported by Kattis, nor can I spot any problems attempting to use it)
• Adds a number of checks for include files (multiple drivers, unknown languages, plus issues that pop up with the new feature of "default" include files)
• Moves mainfile detection from run/ to Language (will be needed for Entrypoint #388)

Default include files are checked in this PR, but NOT supported (I opted not to touch run/ too much in this PR)