Add AI planning for airlifts.

Downside to the current implementation is that whether or not transports
that were purchased last turn will be available for airlift this turn is
arbitrary. This is because transfers are created at the same time as
units are delivered, and units are delivered in an arbitrary order per
CP. If the helicopters are delivered before the ground units they'll
have access to the transports, otherwise they'll be refunded. This will
be fixed later when I rework the transfer requests to not require
immediate fulfillment.

https://github.com/Khopa/dcs_liberation/issues/825

game/theater/controlpoint.py

@@ -344,10 +344,8 @@ class ControlPoint(MissionTarget, ABC):
         if not game.settings.enable_new_ground_unit_recruitment:
             return True
 
-        from game.theater.supplyroutes import SupplyRoute
-
-        for cp in SupplyRoute.for_control_point(self):
-            if cp.can_recruit_ground_units(game):
+        for cp in game.theater.controlpoints:
+            if cp.is_friendly(self.captured) and cp.can_recruit_ground_units(game):
                 return True
         return False