Python: Remove points-to to from ControlFlowNode

Moves the existing points-to predicates to the newly added class
`ControlFlowNodeWithPointsTo` which resides in the `LegacyPointsTo`
module.

(Existing code that uses these predicates should import this module, and
references to `ControlFlowNode` should be changed to
`ControlFlowNodeWithPointsTo`.)

Also updates all existing points-to based code to do just this.

python/ql/examples/snippets/store_none.ql

@@ -10,9 +10,10 @@
  */
 
 import python
+private import LegacyPointsTo
 
 from SubscriptNode store
 where
   store.isStore() and
-  store.getIndex().pointsTo(Value::named("None"))
+  store.getIndex().(ControlFlowNodeWithPointsTo).pointsTo(Value::named("None"))
 select store