this doesn't serve a great purpose yet except to confine construction of
goals to the stack frame of Worker::run() and its child frames. we don't
need this yet (and the goal constructors remain fully visible), but in a
future change that fully removes the current worker loop we'll need some
way of knowing which goals are top-level goals without passing the goals
themselves around. once that's possible we can remove visible goals as a
concept and rely on build result futures and a scheduler built upon them
Change-Id: Ia73cdeffcfb9ba1ce9d69b702dc0bc637a4c4ce6
eldritch horrors
869666cb65