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/src/Imports/FromImportOfMutableAttribute.ql

@@ -12,6 +12,7 @@
  */
 
 import python
+private import LegacyPointsTo
 import semmle.python.filters.Tests
 
 from ImportMember im, ModuleValue m, AttrNode store_attr, string name
@@ -23,7 +24,7 @@ where
   /* variable resulting from import must have a long lifetime */
   not im.getScope() instanceof Function and
   store_attr.isStore() and
-  store_attr.getObject(name).pointsTo(m) and
+  store_attr.getObject(name).(ControlFlowNodeWithPointsTo).pointsTo(m) and
   /* Import not in same module as modification. */
   not im.getEnclosingModule() = store_attr.getScope().getEnclosingModule() and
   /* Modification is not in a test */