Need finer-grained control over PPA dependencies

This would be a very highly welcome change in Bileto as there just needs to be flexibility on setting dependencies. Right now we're wrongly assuming that the overlay is what we want, but there are even cases where we don't want to use the overlay and only use the archive, or like in the snapshot case. Not to mention future uses of Bileto by different teams like cloud etc - those wouldn't want to use the overlay but maybe some other PPA instead.

Adding a task for canonical-devices-system-image for tracking purposes. Having this feature would be the long-term fix for our problem with unity8-session snaps being built with the proposed pocket.

In the meantime, we do have workarounds if needed -- either avoid using the hotfixes silo for building. Or source copy the the packages in that silo to another PPA and then build from there, avoiding bileto.

I think we might want to enforce that silos with development series targets (ie zesty) must be built against -proposed when this feature is implemented, to help guard against people building packages without, and then landing them. Compiler and library updates there tend to be more severe, and if not careful, landing things without building against -proposed can break migrations or end up in release earlier than they should while dependencies are sitting in -proposed.

Since there are many variables that determine when things get through -proposed, and not all packages have autopkgtests, or sufficient ones when they do, I'd be cautious to rely solely on proposed migration blocking things in the situation where one builds a silo without -proposed enabled.