This overlaps a lot with other issues such as issue64. Making the minimal
required core as small as possible is an ongoing concern in the untangling of
dependencies but perhaps not something we should track in this issue.

The Makefile already contains many things that can be commented out without
breaking the build. We should pursue this further to make the absolutely
necessary core of the planner even smaller.

For example, many of the evaluators and open lists can probably be turned into
pluggable components.