Specify name for fetchTarballs

[emilazy]

Not sure if this is done already with other fetchers, so I'm only reporting this most restricted version, but ending up with a bunch of undifferentiated *-sources in the store can be pretty confusing. builtins.fetchTarball actually does support passing a name, though it's undocumented.

[nmattia]

That's a great idea! I didn't know about it. I'll get started.