mostly removing of nodes from the graph.
One result lost:
```
check("submodule.submodule_attr", submodule.submodule_attr, "submodule_attr", globals()) #$ MISSING:prints=submodule_attr
```
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.
The easiest way to implement this was to change the definition of
`module_export` to account for chains of `import *`. We reuse the
machinery from `ImportStar.qll` for this, naturally.
TL;DR: We were missing out on flow in the following situation:
`mod1.py`:
```python
foo = SOURCE
```
`mod2.py`:
```python
from mod1 import *
```
`test.py`:
```python
from mod2 import foo
SINK(foo)
```
This is because there's no node at which a read of `foo` takes place
within `test.py`, and so the added reads make no difference.
Unfortunately, this means the previous test was a bit too simplistic,
since it only looks for module variable reads and writes. Because of
this, we change the test to be a more traditional "all flow" style
(though restricted to `CfgNode`s).
Adds result for `ModuleVariableNode::getARead` corresponding to reads
that go through (chains of) `import *`.
This required a bit of a change to _which_ module variables we define.
Previously, we only included variables that were accessed elsewhere in
the same file, but now we must ensure to also include variables that may
be accessed through `import *`.
