Python: deprecate AstNode.getAFlowNode() and rewrite internal callers

Preparatory refactor for the shared-CFG dataflow migration.

Deprecates the AstNode.getAFlowNode() cached predicate on the public
Python QL API and rewrites all ~140 internal callers across lib/, src/,
test/, and tools/ from `expr.getAFlowNode() = cfgNode` to
`cfgNode.getNode() = expr`, using ControlFlowNode.getNode() which
already exists in Flow.qll.

The predicate itself is preserved (with a deprecation note pointing at
the new pattern) so external users do not experience churn — they can
migrate at their own pace and the AST/CFG hierarchies still get the
intended untangling once the deprecation eventually elapses.

Semantic noop verified by:
- All 361 lib/ + src/ queries compile clean.
- All 122 ControlFlow + PointsTo library-tests pass.
- All 64 dataflow library-tests pass.
- All 113 Variables/Exceptions/Expressions/Statements/Functions/Imports/
  Security/CWE-798/ModificationOfParameterWithDefault query-tests pass.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

python/ql/test/2/library-tests/comprehensions/ConsistencyCheck.ql

@@ -5,5 +5,7 @@
 import python
 
 select count(Comprehension c |
-    count(c.toString()) != 1 or count(c.getLocation()) != 1 or not exists(c.getAFlowNode())
+    count(c.toString()) != 1 or
+    count(c.getLocation()) != 1 or
+    not exists(ControlFlowNode n | n.getNode() = c)
   )

python/ql/test/experimental/import-resolution/importflow.ql

@@ -45,13 +45,15 @@ private class VersionGuardedNode extends DataFlow::Node {
 
   VersionGuardedNode() {
     version in [2, 3] and
-    exists(If parent, CompareNode c | parent.getBody().contains(this.asExpr()) |
+    exists(If parent, CompareNode c, ControlFlowNode litCfg |
+      parent.getBody().contains(this.asExpr()) and
+      litCfg.getNode() = any(IntegerLiteral lit | lit.getValue() = version)
+    |
       c.operands(API::moduleImport("sys")
             .getMember("version_info")
             .getASubscript()
             .asSource()
-            .asCfgNode(), any(Eq eq),
-        any(IntegerLiteral lit | lit.getValue() = version).getAFlowNode())
+            .asCfgNode(), any(Eq eq), litCfg)
     )
   }
 

python/ql/test/library-tests/ControlFlow/PointsToSupport/UseFromDefinition.ql

@@ -9,7 +9,7 @@ Expr assignedValue(Name n) {
 
 from Name def, DefinitionNode d
 where
-  d = def.getAFlowNode() and
+  d.getNode() = def and
   exists(assignedValue(def)) and
   not d.getValue().getNode() = assignedValue(def)
 select def.toString(), assignedValue(def)

python/ql/test/library-tests/ControlFlow/splitting/NodeCount.ql

@@ -8,4 +8,4 @@ where
   not a instanceof ExprStmt and
   a.getScope() = s and
   s instanceof Function
-select a.getLocation().getStartLine(), s.getName(), a, count(a.getAFlowNode())
+select a.getLocation().getStartLine(), s.getName(), a, count(ControlFlowNode n | n.getNode() = a)

python/ql/test/library-tests/PointsTo/global/Global.ql

@@ -3,6 +3,6 @@ private import LegacyPointsTo
 
 from ControlFlowNode f, PointsToContext ctx, Value obj, ControlFlowNode orig
 where
-  exists(ExprStmt s | s.getValue().getAFlowNode() = f) and
+  exists(ExprStmt s | f.getNode() = s.getValue()) and
   PointsTo::pointsTo(f, ctx, obj, orig)
 select ctx, f, obj.toString(), orig

python/ql/test/library-tests/PointsTo/local/LocalPointsTo.ql

@@ -4,6 +4,6 @@ import semmle.python.objects.ObjectInternal
 
 from ControlFlowNode f, ObjectInternal obj, ControlFlowNode orig
 where
-  exists(ExprStmt s | s.getValue().getAFlowNode() = f) and
+  exists(ExprStmt s | f.getNode() = s.getValue()) and
   PointsTo::pointsTo(f, _, obj, orig)
 select f, obj.toString(), orig

python/ql/test/library-tests/dataflow/tainttracking/TestTaintLib.qll

@@ -43,7 +43,7 @@ query predicate test_taint(string arg_location, string test_res, string scope_na
       // TODO: Replace with `hasFlowToExpr` once that is working
       if
         TestTaintTrackingFlow::flowTo(any(DataFlow::Node n |
-            n.(DataFlow::CfgNode).getNode() = arg.getAFlowNode()
+            n.(DataFlow::CfgNode).getNode().getNode() = arg
           ))
       then has_taint = true
       else has_taint = false