Clean up flight plan code.

Split the oversized file into one per plan type. This also moves the
layout responsibility out of the oversized FlightPlanBuilder and into
each flight plan type file.

game/ato/flightplans/planningerror.py

@@ -0,0 +1,5 @@
+from __future__ import annotations
+
+
+class PlanningError(RuntimeError):
+    """Raised when the flight planner was unable to create a flight plan."""