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/coverage-py3/classes.py

@@ -51,7 +51,7 @@ class With_length_hint:
 def test_length_hint():
     import operator
 
-    with_length_hint = With_length_hint()  #$ arg1="SSA variable with_length_hint" func=With_length_hint.__length_hint__
+    with_length_hint = With_length_hint()  #$ arg1="with_length_hint" func=With_length_hint.__length_hint__
     operator.length_hint(with_length_hint)
 
 
@@ -68,5 +68,5 @@ class With_index:
 def test_index():
     import operator
 
-    with_index = With_index()  #$ arg1="SSA variable with_index" func=With_index.__index__
+    with_index = With_index()  #$ arg1="with_index" func=With_index.__index__
     operator.index(with_index)