Python: remove EssaNodes

This commit removes SSA nodes from the data flow graph. Specifically, for a definition and use such as
```python
  x = expr
  y = x + 2
```
we used to have flow from `expr` to an SSA variable representing x and from that SSA variable to the use of `x` in the definition of `y`. Now we instead have flow from `expr` to the control flow node for `x` at line 1 and from there to the control flow node for `x` at line 2.

Specific changes:
- `EssaNode` from the data flow layer no longer exists.
- Several glue steps between `EssaNode`s and `CfgNode`s have been deleted.
- Entry nodes are now admitted as `CfgNodes` in the data flow layer (they were filtered out before).
- Entry nodes now have a new `toString` taking into account that the module name may be ambigous.
- Some tests have been rewritten to accomodate the changes, but only `python/ql/test/experimental/dataflow/basic/maximalFlowsConfig.qll` should have semantic changes.
- Comments have been updated
- Test output has been updated, but apart from `python/ql/test/experimental/dataflow/basic/maximalFlows.expected` only `python/ql/test/experimental/dataflow/typetracking-summaries/summaries.py` should have a semantic change. This is a bonus fix, probably meaning that something was never connected up correctly.

python/ql/test/experimental/dataflow/basic/maximalFlowsConfig.qll

@@ -1,3 +1,4 @@
+import python
 import semmle.python.dataflow.new.DataFlow
 private import semmle.python.dataflow.new.internal.DataFlowPrivate as DataFlowPrivate
 
@@ -11,14 +12,12 @@ class MaximalFlowsConfig extends DataFlow::Configuration {
   override predicate isSource(DataFlow::Node node) {
     node instanceof DataFlow::ParameterNode
     or
-    node instanceof DataFlow::EssaNode and
-    not exists(DataFlow::EssaNode pred | DataFlow::localFlowStep(pred, node))
+    node instanceof DataFlow::LocalSourceNode
   }
 
   override predicate isSink(DataFlow::Node node) {
     node instanceof DataFlowPrivate::ReturnNode
     or
-    node instanceof DataFlow::EssaNode and
-    not exists(node.(DataFlow::EssaNode).getVar().getASourceUse())
+    not DataFlowPrivate::LocalFlow::localFlowStep(node, _)
   }
 }