Adding the default mod again.

We did get at least 4 reports flare being unplayable because of the default mod being missing.
Probably a lot of problems relating to the default mod being treated special go unreported.
Hence let's re-add the default mod to the game data.

Flare originally was trying to be simple. This may not be true any more
for some parts of the code (optimizing and error-handling contradict
being simple), but it should be simple for the users to play.

We need to discuss whether we keep the special treatment in the engine or roll it back as well.