add ability to reference local pack in overrides

[bdew]

This lets you write something like that in mappings.txt

model.transports.medium.wagon.molr=~graphics.jar/transports/Wagon_main.wom?tentMat.texture=~[local]/wagon_SA.png

Which will load wagon model from graphics.jar but override one of the textures from the current pack.

[local] will be replaced by the name of the pack at runtime.