Rename constants to populations

This is a better name for the module because it’s:
- not just constants, there’s a method in here now
- only stuff to do with populations, not other kinds of constants

tests/app/broadcast_areas/test_broadcast_area.py

@@ -4,7 +4,7 @@ from app.broadcast_areas import (
     BroadcastAreasRepository,
     broadcast_area_libraries,
 )
-from app.broadcast_areas.constants import CITY_OF_LONDON
+from app.broadcast_areas.populations import CITY_OF_LONDON
 
 
 def test_loads_libraries():