more progress; initial stab at calling hscMain from the driver

Summary:

  * in normal mode, the driver calls hscMain directly.  The
    compilation manager is not involved at all.  This bit is almost
    there, just waiting for hscMain to catch up.

  * in `ghc --make' mode, the driver will hand off control to the
    compilation manager, and provide the `compile' interface.  This
    bit isn't there yet.

  * in `ghc --interactive' mode, the driver will hand off control
    to the user interface (which doesn't exist yet), which will in
    turn invoke the compilation manager, which in turn invokes the
    driver again through the `compile' interface.  None of this
    is there yet.