ghc: greatly simplify the GHC compiler wrapper

The wrapper script accumulated some cruft over the last couple of months
because we did changes in freaky ways to avoid triggering re-builds of all
Haskell packages. Most of these kludges have been thrown out now.

This patch doesn't change the behavior of the wrapper except for one thing: the
internal helper scripts "ghc-get-packages.sh" and "ghc-packages.sh" are no
longer installed in the bin directory of the generated derivation.