Allow type checking modules assigned to variables

[JukkaL]

Currently modules are special names. Assignment does not propagate this:

import os
os.getcwd() # ok
x = os
x.getcwd() # error

Aliases to modules should be supported.

[ilevkivskyi]

This is now fixed on master. (Except for documenting this feature, but there is a separate issue #3494 for this )