Rework derivation input resolution

I refactored the way that input resolution works in `DerivationGoal`. To
be honest, it is probably unclear to the reader whether this new way is
better or worse. I suppose *intrinsic* motivation, I can say that

- the more structured use of `inputGoal` (a local variable) is better
  than the shotgrun approach with `inputDrvOutputs`

- A virtual `waiteeDone` was a hack, and now it's gone.

However, the *real* motivation of this is not the above things, but that
it is needed for my mammoth refactor fixing #11897 and #11928.

It is nice that this step could come first, rather than making that
refactor even bigger.

src/libstore/build/worker.cc

@@ -552,4 +552,9 @@ GoalPtr upcast_goal(std::shared_ptr<DrvOutputSubstitutionGoal> subGoal)
     return subGoal;
 }
 
+GoalPtr upcast_goal(std::shared_ptr<DerivationGoal> subGoal)
+{
+    return subGoal;
+}
+
 }