This method does *not* create a new type of goal. We instead just make
`DerivationGoal` more sophisticated, which is much easier to do now that
`DerivationBuildingGoal` has been split from it (and so many fields are
gone, or or local variables instead).
This avoids the need for a secondarily trampoline goal that interacted
poorly with `addWantedOutputs`. That, I hope, will mean the bugs from
before do not reappear.
There may in fact be a reason to introduce such a trampoline in the
future, but it would only happen in conjunction with getting rid of
`addWantedOutputs`.
Restores the functionality (and tests) that was reverted in
f4f28cdd0e.
