Allow circular imports

[mostafa-abdullah]

Added support for circular imports.

Changes:

• Run the pre-analysis on an imported no more than once.
• Cache the imported context before running the inference algorithm on it, so the next time it is encountered, the cached one will be returned.

Behavior:

Works like it does in Python. When a circular import occurs, only parts of the module that appear before the import statements are considered. For example the following will raise an error:

A.py

import B

def f():...

B.py

import A

A.f()

# Error: A has no attribute f

However, the following is supported:

A.py

def f():...

import B

B.py

import A

A.f()