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.
This takes care of scoping for type parameters on functions, but not
type aliases or classes.
For classes, the _type parameters_ now have the correct `Class` as scope,
but all their child nodes do not (e.g. the `Name` inside a `TypeParameter`).
This has to do with how the `py_scopes` relation is emitted by the extractor,
since `Name`s are expressions.
Mostly to highlight that with flow-summary modeling, we don't expect
taint for a lot of these.
I aslo opted to make `finditer()` tainted for consistency.
In the upgrade direction, we simply do nothing.
In the downgrade direction, we remove the two new relations, and
also any `Stmt` nodes corresponding to `TypeAlias` nodes.
For these, I opted for a placement that would cause as few changes to the
dbscheme as possible. This puts the new `type_parameters` fields as the
last field on function and class definitions.
Factor out use-use flow in order to do this.
Also improve names and comments.
I also wanted to change the types in `difinitionFlowStep`, but
that broke the module instantiation.
