Big refactoring to clear the way for NixOS integration

Nixpkgs checkout place moved to /opt/ceh/nixpkgs.

Use the nix and perl from the nixpkgs baseline, not from the
predownloaded nix binary bundle.

Get rid of derivation hashes, they were an overcomplication and it can
actually happen in corner cases that two different derivation results in
the same output path.
Merge branch 'master' of github.com:errge/ceh

Conflicts:
	lib/Packages/GHC.pm