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

game/squadrons/squadrondefloader.py

@@ -0,0 +1,68 @@
+from __future__ import annotations
+
+import logging
+from collections import defaultdict
+from pathlib import Path
+from typing import Iterator, Tuple, TYPE_CHECKING
+
+from game.dcs.aircrafttype import AircraftType
+from .squadrondef import SquadronDef
+
+if TYPE_CHECKING:
+    from game import Game
+    from game.coalition import Coalition
+
+
+class SquadronDefLoader:
+    def __init__(self, game: Game, coalition: Coalition) -> None:
+        self.game = game
+        self.coalition = coalition
+
+    @staticmethod
+    def squadron_directories() -> Iterator[Path]:
+        from game import persistency
+
+        yield Path(persistency.base_path()) / "Liberation/Squadrons"
+        yield Path("resources/squadrons")
+
+    def load(self) -> dict[AircraftType, list[SquadronDef]]:
+        squadrons: dict[AircraftType, list[SquadronDef]] = defaultdict(list)
+        country = self.coalition.country_name
+        faction = self.coalition.faction
+        any_country = country.startswith("Combined Joint Task Forces ")
+        for directory in self.squadron_directories():
+            for path, squadron_def in self.load_squadrons_from(directory):
+                if not any_country and squadron_def.country != country:
+                    logging.debug(
+                        "Not using squadron for non-matching country (is "
+                        f"{squadron_def.country}, need {country}: {path}"
+                    )
+                    continue
+                if squadron_def.aircraft not in faction.aircrafts:
+                    logging.debug(
+                        f"Not using squadron because {faction.name} cannot use "
+                        f"{squadron_def.aircraft}: {path}"
+                    )
+                    continue
+                logging.debug(
+                    f"Found {squadron_def.name} {squadron_def.aircraft} "
+                    f"{squadron_def.role} compatible with {faction.name}"
+                )
+
+                squadrons[squadron_def.aircraft].append(squadron_def)
+        return squadrons
+
+    @staticmethod
+    def load_squadrons_from(directory: Path) -> Iterator[Tuple[Path, SquadronDef]]:
+        logging.debug(f"Looking for factions in {directory}")
+        # First directory level is the aircraft type so that historical squadrons that
+        # have flown multiple airframes can be defined as many times as needed. The main
+        # load() method is responsible for filtering out squadrons that aren't
+        # compatible with the faction.
+        for squadron_path in directory.glob("*/*.yaml"):
+            try:
+                yield squadron_path, SquadronDef.from_yaml(squadron_path)
+            except Exception as ex:
+                raise RuntimeError(
+                    f"Failed to load squadron defined by {squadron_path}"
+                ) from ex