Merge the get/download methods on the Source trait

Merge the get/download methods on the Source trait

Nothing currently implements the ability to more efficiently download a set of
packages at any one point in time, and the download/get distinction isn't really
used at all. We can always refactor later, but currently there's no benefit, nor
can it really be seen what the possible benefit is, so let's just merge these
two methods into one and have them operate on one id at a time.