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.
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.
copy, pop, get, getitem, setdefault
Also add read steps to taint tracking.
Reading from a tainted collection can be done in two situations:
1. There is an acces path
In this case a read step (possibly from a flow summary)
gives rise to a taint step.
2. There is no access path
In this case an explicit taint step (possibly via a flow
summary) should exist.
Since we can analyze operator.py from Python3, but not in Python 2
(since it's implemented in C), we get a difference for the index tests.
note: `operator.length_hint` is only available in Python 3.4 and later,
so would always fail under Python 2.
Adjusted `tracked.ql`
- no need to annotate results on line 0
this could happen for global SSA variables
- no need to annotate scope entry definitons
they look a bit weird, as the annotation goes on the
line of the function definition.
This is a bit of an edge case, but allowed. Since we currently don't
provide information on positional only arguments, we can't do much to
solve it right now.
Similar to the Ruby changes from
https://github.com/github/codeql/pull/11461
I feel the change to `DataFlowFunciton.getParameter` where we use
`not exists(func.getArgByName(_))` is not very great, but I was not allowed
to use `not exists(this.getParameter(any(ParameterPosition _).isKeyword(_)))`
because of negative recursion.
validTest.py did not pass, since we use `SINK3_F`.
I initially tried swapping the order
```
args = (arg1, arg2) # $ arg1 arg2 func=starargs_only
more_args = (arg4, arg3)
starargs_only(*args, *more_args)
```
But then asked myself, what is it _actually_ we're testing here? and it
seems to be the way we handle multiple *args arguments in the same call,
so I converted the test to be that instead! (and it matches what we do
in test_stararg_mixed)
