Conditonally define module after initialization to None

[JukkaL]

Mypy complains about this code, even though it's valid Python:

if x:
    m = None
else:
    import m

The most obvious way to fix this would be to add support for "module types" for specific modules. In the above example, m would be a variable with a partial None type initially. The import statement would switch the partial type to the type of module m.

(This is derived from discussion in #649.)

[JukkaL]

Module types would also make module aliases via assignment work:

import m
alias = m
alias.foo()  # Mypy currently complains about this