Liquid: add getMinAdjacentSourcesToFormSource(), remove circular dependency between Liquid and Water

this unpleasantly-named method allows controlling the source-forming behaviour of liquids by changing the required number of adjacent sources that must be present in order for a new source to form. This allows stuff like non-infinite water.

src/block/Water.php

@@ -46,6 +46,10 @@ class Water extends Liquid{
 		return 5;
 	}
 
+	public function getMinAdjacentSourcesToFormSource() : ?int{
+		return 2;
+	}
+
 	public function onEntityInside(Entity $entity) : bool{
 		$entity->resetFallDistance();
 		if($entity->isOnFire()){