Split main/GHC into GHC and GhcMake

There are two things going on in main/GHC.hs.
  * It's the root module of the GHC package
  * It contains lots of stuff for --make
It is also gigantic (2.7k lines)

This patch splits it into two
  * GHC.hs is the root module for the GHC package
      (1.3k lines)
  * GhcMake.hs contains the stuff for --make
      (1.4k lines)

Happily the functional split divided it almost
exactly in half.

This is a pure refactoring.  There should be no
behavioural change.