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/commander/packagefulfiller.py

@@ -5,6 +5,8 @@ from collections import defaultdict
 from typing import Dict, Iterable, Optional, Set, TYPE_CHECKING
 
 from game.ato.airtaaskingorder import AirTaskingOrder
+from game.ato.closestairfields import ObjectiveDistanceCache
+from game.ato.flightplans.flightplanbuilder import FlightPlanBuilder
 from game.ato.flighttype import FlightType
 from game.ato.package import Package
 from game.commander.missionproposals import EscortType, ProposedFlight, ProposedMission
@@ -17,8 +19,6 @@ from game.settings import Settings
 from game.squadrons import AirWing
 from game.theater import ConflictTheater
 from game.threatzones import ThreatZones
-from game.ato.closestairfields import ObjectiveDistanceCache
-from game.ato.flightplan import FlightPlanBuilder
 
 if TYPE_CHECKING:
     from game.ato import Flight