lookaside: Handle downloading of source files
    
    This is quite a large chunk of code that is moving from pyrpkg.Commands
    to the new pyrpkg.lookaside.CGILookasideCache.
    
    In addition, we now don't fork a call to the curl command any more, but
    instead use pycurl. This allows the download method to be entirely
    tested.
    
    Downstreams can now also easily use their own URL format, which should
    make the CentOS folks happier, as well as make it easier when we want to
    add the hashtype to the URL in Fedora. (as part of the move away from
    md5 hashes)
    
    As part of the move to pycurl, we are losing the download progress
    indication, but it will make a come-back in a future commit.