[entropy.client.client] move clear_cache() to Client
[entropy.client] rename _filter_available_repositories() to filter_repositories()
[entropy.client.cache] drop _all_repositories_hash, duplicate of _repositories_hash()
[entropy.client.cache] drop _selective_repositories_hash()
[entropy.client.cache] improve _repositories_hash() robustness against collisions
[entropy.client] rename _repositories_hash() to repositories_checksum()
[entropy.client] add missing imports
[entropy.client] drop _get_available_packages_hash()
[entropy.client] drop CacheMixin class
[entropy.qa] rename ErrorReportInterface to ErrorReport, it's cleaner