Change squadrons to operate out of a single base.

https://github.com/dcs-liberation/dcs_liberation/issues/1145

Currently this is fixed at the start of the campaign. The squadron
locations are defined by the campaign file.

Follow up work:

* Track aircraft ownership per-squadron rather than per-airbase.
* UI for relocating squadrons.
* Ferry missions for squadrons that are relocating.
* Auto-relocation (probably only for retreat handling).

Fixes https://github.com/dcs-liberation/dcs_liberation/issues/1138

gen/naming.py

@@ -1,12 +1,16 @@
+from __future__ import annotations
+
 import random
 import time
-from typing import List, Any
+from typing import List, Any, TYPE_CHECKING
 
 from dcs.country import Country
 
 from game.dcs.aircrafttype import AircraftType
 from game.dcs.unittype import UnitType
-from gen.flights.flight import Flight
+
+if TYPE_CHECKING:
+    from gen.flights.flight import Flight
 
 ALPHA_MILITARY = [
     "Alpha",