Use the actual Country type instead of the name.

We want other pieces of country information (in particular the short
names). This cleans up a lot of code anyway.

As an added bonus, this now catches squadrons that used invalid names
which would previously be passed through to pydcs and... then I don't
know what would happen.

game/dcs/countries.py

@@ -0,0 +1,9 @@
+from dcs.countries import country_dict
+from dcs.country import Country
+
+
+def country_with_name(name: str) -> Country:
+    for country in country_dict.values():
+        if country.name == name:
+            return country()
+    raise KeyError(f"No country found named {name}")