Repo and RepoDict.

Class skeletons with a test.
removal: pruning imports in dnf.yum.__init__
removing: trim down dnf.yum.packages.

This is 1.6k lines of legacy code down.
remove: _YumCostExclude, unused.
strip things out of dnf/yum/__init__.py
move yum.Base to yum.base.Base.

It is imported all the time otherwise and creates the dreaded import
cycles.