Reorganize planner code moved in
b60c39759908
It seems modules are better defined like this instead of the original
split.
Per complaints from David Rowley as well as Amit Langote's self review.
Discussion: https://postgr.es/m/CAKJS1f988rsyhwvLgfT-y1UCYUfXDOv67ENQk=v24OxhsZOzZw@mail.gmail.com