This makes the attribute (a) consistent with "doCheck" and friends and (b)
avoids the double negation "noHaddock = false" meaning "doHaddock = true".
Fixes https://github.com/NixOS/cabal2nix/issues/63.
deepOverride turns out to be completely unfeasible for non-trivial overrides.
Nix evaluates for an eternity, and then comes back saying:
Too many heap sections: Increase MAXHINCR or MAX_HEAP_SECTS
nix-instantiate killed by signal 6
The hand-written deep-override, on the other hand, performs the job in a
fraction of a second, no problem.
All bow to Russell O'Connor!
