Python: Fix query tests

Mostly just adding `private import LegacyPointsTo`. Sometimes getting
rid of other imports that are superceded by that module.

python/ql/src/Variables/Undefined.qll

@@ -1,6 +1,7 @@
 import python
 import Loop
 import semmle.python.dataflow.TaintTracking
+private import LegacyPointsTo
 
 /** A marker for "uninitialized". */
 class Uninitialized extends TaintKind {

python/ql/src/Variables/UndefinedPlaceHolder.ql

@@ -13,6 +13,7 @@
 
 import python
 import Variables.MonkeyPatched
+private import LegacyPointsTo
 
 /* Local variable part */
 predicate initialized_as_local(PlaceHolder use) {

python/ql/src/Variables/UnusedParameter.ql

@@ -14,6 +14,7 @@
 
 import python
 import Definition
+private import LegacyPointsTo
 
 predicate unused_parameter(FunctionValue f, LocalVariable v) {
   v.isParameter() and