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/test/library-tests/objects/Strings.ql

@@ -1,5 +1,6 @@
 import python
+private import LegacyPointsTo
 
-from StringObject s, ControlFlowNode f
+from StringObject s, ControlFlowNodeWithPointsTo f
 where f.refersTo(s)
 select f.getLocation().toString(), s.getText()