Re-enable ArtifactBlacklist
[refactor] Move implementation of resolver to impl package
[refactor] Move implementation of repository to impl package
[refactor] Move implementation of configurators to impl package
[refactor] Move implementation of dependency extractors to impl package
[refactor] Move RpmDb to appropriate impl package

RpmDb is used by resolver only.  It is not considered as part of
public API and it doesn't make sense to create a separate package
for it.
[refactor] Remove AbstractResolver
[refactor] Make some implementation classes as non-public