Add in-memory hash for caching rpmdb dependency lookups

Add in-memory hash for caching rpmdb dependency lookups
- worst case behavior for uncached dependency lookups can be disastrous,
  eg > 35s vs < 1s on my laptop for trying to remove /bin/sh provider
- we only bother caching rpmdb lookups, the other cases plenty fast already
- using in-memory cache avoids nasty in vs out of chroot issues with
  temporary db files, which otherwise were about as fast