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/squadrons/squadron.py

@@ -9,9 +9,9 @@ from typing import Optional, Sequence, TYPE_CHECKING
 from faker import Faker
 
 from game.ato import Flight, FlightType, Package
-from game.ato.flightplan import FlightPlanBuilder
 from game.settings import AutoAtoBehavior, Settings
 from .pilot import Pilot, PilotStatus
+from ..ato.flightplans.flightplanbuilder import FlightPlanBuilder
 from ..db.database import Database
 from ..utils import meters