Improve logging while uploading individual files to artifacts

Without API changes in dput-ng, we can't both avoid duplicate logging from dput-ng and also make the logging be a useful indication of progress, so this means you get a quick series of "Uploading {name}" lines from dput-ng itself followed by a slower series of "Uploading {path} to {url}" lines from the Debusine client. That's better than a mysterious pause, though.

Fixes: #839 (closed)