[MODELDEF] Directive to only replace IWAD assets

[Rachael]

Not sure how doable this is, given the existing limitations of MODELDEF, but here goes. The main purpose of this would be to allow loading model packs, but still enable classic (DEHACKED+sprite-replacement) PWAD use.

[Graf Zahl]

Tricky. There is no guarantee that all rotations of a frame come from the same WAD.