Dedup purchase requests.

Since the theater commander runs once per campaign action, missions that
do not have aircraft available may be checked more than once a turn.
Without deduping requests this can lead to cases where the AI buys
dozens of tankers on turn 0.

Fixes https://github.com/dcs-liberation/dcs_liberation/issues/1470

game/transfers.py

@@ -751,6 +751,6 @@ class PendingTransfers:
             # aesthetic.
             gap += 1
 
-        self.game.procurement_requests_for(self.player).append(
+        self.game.coalition_for(self.player).add_procurement_request(
             AircraftProcurementRequest(control_point, FlightType.TRANSPORT, gap)
         )