use pkgCache::VS instead of pkgDepCache::VS()

use pkgCache::VS instead of pkgDepCache::VS()

The later just calls the earlier, but the later needs the fullblown
dependency cache to be initialized, which is a very costly operation and
isn't done anymore that early in the run as we would need to throw away
and rebuild it again after we got all the information about source pkgs.

As we end up with a nullptr for the pkgDepCache, we use a slightly
longer calling convention to make sure that we use the pkgCache
directly, avoiding nullptr induced segfaults and costly operations.

Git-Dch: Ignore
Reported-By: Balint Reczey <balint@balintreczey.hu>