Added option to skip computation of larger conflict sets.

Sometimes, the computation of larger conflict sets may take forever; add an option to skip this computation when needed.
See snapshot.debian.net between 12/3/2005 and 6/10/2005 for an example (http://snapshot.debian.org/archive/debian/20051006T000000Z/dists/testing/main/binary-i386/Packages.gz and similar ones)