* Fix dep_check() so that it doesn't expand virtual blockers since the

* Fix dep_check() so that it doesn't expand virtual blockers since the
  un-expanded virtual atom is more useful for maintaining a cache of
  blocker atoms.

* Expand virtual blockers in depgraph.validate_blockers(), since it's
  not done by dep_check() anymore.

* If blocker data from the graph is available, use it to validate the
  blocker cache and update the cache if it seems invalid.

* Make BlockerCache._load() more tolerant to installs/uninstalls so
  so that cache isn't rebuilt every time.

svn path=/main/trunk/; revision=10041