[entropy.client.dep] fix format string typo
[entropy.client.cache] drop methods used by the dep module only
[entropy.core] when clearing repository cache, don't clear the whole cache

This is no longer needed since cache keys are now properly generated
[entropy.cache] drop EntropyCacher.CACHE_IDS['world_update']
[entropy.cache] drop EntropyCacher.CACHE_IDS['filter_satisfied_deps']
[entropy.cache] drop EntropyCacher.CACHE_IDS['library_breakage']
[entropy.cache] drop EntropyCacher.CACHE_IDS['depends_tree']
[entropy.cache] drop EntropyCacher.CACHE_IDS['check_package_update']
[entropy.cache] drop EntropyCacher.CACHE_IDS['dep_tree']
[entropy.cache] drop EntropyCacher.CACHE_IDS['atom_search']
[entropy.cache] drop EntropyCacher.CACHE_IDS['atom_match']
[entropy.client.settings] drop repos_installed{_keys,} settings metadata

Load the related data ondemand whenever it is needed, save SystemSettings
load time and avoid problems with stale information.
[entropy.client.dep] drop unused imports
[todo] update TODO