Move sage/graphs/planarity/ to sage/graphs/planarity_c/ #13363
Description
When running doctests with #12659 applied, an "ImportWarning: Not importing directory" is raised due to there being a sage/graphs/planarity.so and sage/graphs/planarity directory. This moves the C source code into a planarity_c directory.
Component: graph theory
Author: Mike Hansen
Reviewer: John Palmieri
Merged: sage-5.3.beta2
Issue created by migration from https://trac.sagemath.org/ticket/13363