Draft: dgit: download-unfetched-origs: Try incoming.d.o

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1111728

Would you take a look at the 2nd-last commit, "dgit: download-unfetched-origs: Try incoming.d.o", and review it for naming and documentation?

I don't need code review of the whole branch; I think I'm pretty happy with the implementation approach, after several false starts.

Draft because:

• Needs fix for whole-archive path too
• Local every-commit tests haven't finished yet
• Want review from @spwhitton re naming and docs