Story #1150: As a user, I can lazily fetch repositories
Update the Importer base class to support the streamer getting a configured downloader.
Add something like:
Importer.get_downloader(self, config, catalog_entry).
This method is called by the streamer to acquire a fully configured downloader as needed. This method could have a default implementation that creates a downloaded using the standard importer config. This implementation could provide a sufficient implementation for most importers participating in lazy downloading.