Packages cleanup, and allow new packages to be loaded with :set again

This cleans up the package subsystem a little.  There are some
changes to the GHC API as a result.

  - GHC.init and GHC.initFromArgs are no longer necessary.

  - GHC.newSession takes the root of the GHC tree as an argument
    (previously passed to GHC.init).

  - You *must* do GHC.setSessionDynFlags after GHC.newSession,
    this is what loads the package database.

  - Several global vars removed from SysTools

  - The :set command in GHCi can now cause new packages to be loaded,
    or can hide/ignore existing packages.