ghc-7.0.1: fix download URL
all-packages.nix: cosmetic
haskell-binary: remove obsolete version 0.6.0.0
haskell-deepseq: build this package with the native Cabal version

This is necessary because newer versions of Cabal depend on deepseq,
causing an infinite recursion.
- Disable all traces of Haskell Platform.

- Drop all intermediate GHC releases.

- Recurse into all GHC package sets to improve error detection during
  the refactoring.