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.
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`.
This required a minor change in the type tracker implementation, but
apart from that no other changes appear to be needed. Seems to clean
up the test output quite a bit.
