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/lib/semmle/python/Metrics.qll

@@ -1,4 +1,5 @@
 import python
+private import LegacyPointsTo
 
 /** The metrics for a function */
 class FunctionMetrics extends Function {
@@ -59,7 +60,7 @@ class FunctionMetrics extends Function {
     not non_coupling_method(result) and
     exists(Call call | call.getScope() = this |
       exists(FunctionObject callee | callee.getFunction() = result |
-        call.getAFlowNode().getFunction().refersTo(callee)
+        call.getAFlowNode().getFunction().(ControlFlowNodeWithPointsTo).refersTo(callee)
       )
       or
       exists(Attribute a | call.getFunc() = a |