dependency analysis

Hello everyone, Here's a very-very-very alpha, not-yet-for-general-use patch that enables dependencies in Cabal. Basically, all that works right now is building GHC libraries. I haven't implemented compiling executables yet. But each module is compiled one at a time, and each is scanned for the modules it depends on, and everything is built in the correct order. It still needs lots of work, though :) Pete