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.
I have a vague recollection that ``DownloadReport`` has something like a ``from_request`` class method that would be useful here and below.