This is a very simple mod. It does exactly two things:
1. Overrides all five vanilla village town_centers so they contain only the zombie variant.
2. Waits until ENTITY_LOAD and then uses MobEntity.convertTo()to convert any Villagers marked Structure, Chunk_Generation, or Natural into Zombie Villagers.
Or, put simply, it makes sure that all Villages spawn as Zombie Villages, and it makes sure that all naturally spawning Villagers turn into Zombie Villagers, while leaving Cured, Bred, or Summoned Villagers alone.