Python: Use ImportExp instead of SSA nodes

This also reverts the previous commit.
It should be squashed with that one, but for now we keep the history,
so we can track the performance tests.

Author: yoff

python/ql/src/semmle/python/dataflow/new/TypeTracker.qll

@@ -274,7 +274,7 @@ class TypeTracker extends TTypeTracker {
    * heap and/or inter-procedural step from `nodeFrom` to `nodeTo`.
    */
   pragma[inline]
-  TypeTracker step(Node nodeFrom, Node nodeTo) {
+  TypeTracker step(LocalSourceNode nodeFrom, Node nodeTo) {
     exists(StepSummary summary |
       StepSummary::step(nodeFrom, nodeTo, summary) and
       result = this.append(summary)

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

@@ -46,7 +46,7 @@ Node importNode(string name) {
       or
       name = alias.getValue().(ImportExpr).getImportedModuleName()
     ) and
-    result.(EssaNode).getVar().(AssignmentDefinition).getSourceVariable() = var
+    result.asExpr() = alias.getValue()
   )
   or
   // Although it may seem superfluous to consider the `foo` part of `from foo import bar as baz` to