Integrate yum importer w/ alteranate content sources

The main change is to add an *alternate* module that provides a *Packages* class that is integrated with Alternate Content Sources.  This class is API compatible with *packages.Packages* which is used by the yum synchronization module to download packages.  Taking this approach so we can stage the integration as we want to support additional content types.
Is it possible to pass a generator of requests instead of a list? The original ``packages.Packages`` class does that. Doing so will help keep RAM use under control.