Renamed get_sources to get_source_manifests

To help avoid confusion with _get_sources, which can now become get_sources.

I also delayed the execution of get_download_size in print_transactions so that it only has to run if a summary is needed, and also because it's i/o heavy and can cause a large delay before the transaction list gets printed. Now, that delay is spread out among each transaction individually while the transaction list is being displayed (since the download status gets computed as each item is printed anyway).