Support resumable downloads

I think I've seen this request before, but I can't find it now.

Packages/dependencies could get pretty large and the client may not always have the bandwidth to download all of it at once. Would be nice to be able to resume downloads, perhaps using BITS service.

The issue does not name files, tests, or an entry point. Start by tracing the client’s package and dependency download path, then investigate how resumable transfers could use the Windows BITS service. Done means interrupted large downloads can resume without restarting from the beginning.

Written by the indexing model from the issue text.