Move mission generation code into game.

Operation has been renamed MissionGenerator and is no longer a static
class.

game/ato/package.py

@@ -1,16 +1,20 @@
+from __future__ import annotations
+
 import logging
 from collections import defaultdict
 from dataclasses import dataclass, field
 from datetime import timedelta
-from typing import List, Optional, Dict
+from typing import List, Optional, Dict, TYPE_CHECKING
 
 from game.ato import Flight, FlightType
 from game.ato.packagewaypoints import PackageWaypoints
-from game.theater import MissionTarget
 from game.utils import Speed
 from gen.flights.flightplan import FormationFlightPlan
 from gen.flights.traveltime import TotEstimator
 
+if TYPE_CHECKING:
+    from game.theater import MissionTarget
+
 
 @dataclass
 class Package: