Hackage4Nix.hs: add missing haskell-platform releases to the list of files we cannot re-generate
Hackage4Nix.hs: drop the notion of a "patched package"

That list was empty all the time anyway.
Hackage4Nix.hs: add non-Hackage package "flapjax" to the list of files that cannot be re-generated
Hackage4Nix.hs: fail with a meaningful error message when a package is considered for updating, but it cannot be found in the Hackage database
Hackage4Nix.hs: consider files for updating regardless of how they fetch their sources

Just because a particular version of package is being fetched from
Github, it doesn't mean that future versions won't appear on Hackage
that we might want to update to.