Move the Vertex Pathfinder to its own helper class
The vertex pathfinder was implemented directly in CCmpPathfinder, instead of being a separate helper like the hierarchical pathfinder or the long pathfinder.
This moves it to its own helper VertexPathfinder, which gets us ready for D14 and pathfinder threading. Some struct definitions need to be moved around.
Differential Revision: https://code.wildfiregames.com/D1855