It turns out not to be the right solution for general private
dependencies and is just complicated. However we keep qualified
goals, just much simpler. Now dependencies simply inherit the
qualification of their parent goal. This gets us closer to the
intended behaviour for the --independent-goals feature, and for
the simpler case of private dependencies for setup scripts.

When not using --independent-goals, the solver behaves exactly as
before (tested by comparing solver logs for a hard hackage goal).
When using --independent-goals, now every dep of each independent
goal is qualified, so the dependencies are solved completely
independently (which is actually too much still).