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 *`.
