all-packages.nix: provide a variant of Darcs that includes the executable, but not the library

The reduced 'darcs' derivation is supposed to be run-time independent of the
Haskell compiler. Unfortunately, this isn't quite true yet because the HTTP
package bakes its own store path into the library. This causes Darcs -- which
links the HTTP library statically -- to depend on 'HTTP', which in turn depends
on lot of other Haskell stuff. Patching HTTP to get rid of that reference looks
feasible, though.
darcs: build against the latest versions of the 'tar' and 'haskeline' dependencies