Python: Make ParameterNode a CfgNode

Add a step from that `CfgNode` to the corresponding `EssaNode`.
The intended effect is seen in `ImpliesDataflow.expected`.
The efeect seen in other `.expected`-files is that parameter nodes
change type, that the extra steps are seen, and that flow from
`EssaVar`s is mirrored in flow from `CfgNode`s.
There is one surprise, which is the `.0` node in
`coverage/localFlow.expected`.

python/ql/src/semmle/python/dataflow/new/internal/DataFlowPrivate.qll

@@ -139,6 +139,11 @@ module EssaFlow {
       contextManager.strictlyDominates(var)
     )
     or
+    exists(ParameterDefinition pd |
+      nodeFrom.asCfgNode() = pd.getDefiningNode() and
+      nodeTo.asVar() = pd.getVariable()
+    )
+    or
     // First use after definition
     //   `y = 42`
     //   `x = f(y)`

python/ql/src/semmle/python/dataflow/new/internal/DataFlowPublic.qll

@@ -176,21 +176,21 @@ ExprNode exprNode(DataFlowExpr e) { result.getNode().getNode() = e }
  * The value of a parameter at function entry, viewed as a node in a data
  * flow graph.
  */
-class ParameterNode extends EssaNode {
-  ParameterNode() { var instanceof ParameterDefinition }
+class ParameterNode extends CfgNode {
+  ParameterDefinition def;
+
+  ParameterNode() { node = def.getDefiningNode() }
 
   /**
    * Holds if this node is the parameter of callable `c` at the
    * (zero-based) index `i`.
    */
-  predicate isParameterOf(DataFlowCallable c, int i) {
-    var.(ParameterDefinition).getDefiningNode() = c.getParameter(i)
-  }
+  predicate isParameterOf(DataFlowCallable c, int i) { node = c.getParameter(i) }
 
   override DataFlowCallable getEnclosingCallable() { this.isParameterOf(result, _) }
 
   /** Gets the `Parameter` this `ParameterNode` represents. */
-  Parameter getParameter() { result = var.(ParameterDefinition).getParameter() }
+  Parameter getParameter() { result = def.getParameter() }
 }
 
 /**