Names that start with an underscore should not be included

[asmeurer]

Unless __all__ is defined, names that start with an underscore aren't included with import *. Right now they aren't properly filtered in the static importer.

[asmeurer]

There's a subtlety with recursive imports. If we have

# a.py
_x = 1
__all__ = ['_x']

# b.py
from .a import *

# c.py
from .b import *

then _x should be marked as imported in b.py, but not in c.py.