This is the entry point to actually building the modules in a package. It doesn't actually do much itself, most of the work is delegated to compiler-specific actions. It does do some non-compiler specific bits like running pre-processors.

There's some stuff to do with generating makefiles which is a well hidden feature that's used to build libraries inside the GHC build system but which we'd like to kill off and replace with something better (doing our own dependency analysis properly).