Fix nested namespace package import causing import-error

[chrisphilip322]

Fixes / new features

• Importing a module which is in more than one namespace package, eg foo.bar.baz where foo and bar are both namespace packages, causes an error to be raised and no module to be found
• This merge request only addresses half of the issue; 6099e85
• This specific issue is also documented here Nested namespace packages: No name in module with python2.7 pylint#1686
• Also added a test that breaks without this change, and tested in 2.7.14 and 3.6.2

[brycepg]

I'm seeing astroid importing namespace packages properly on master:

mkdir -p foo/bar/baz
touch foo/bar/baz/__init__.py

import astroid
code = """
import foo.bar.baz
foo.bar.baz #@
"""
astroid.extract_node(code).inferred()

Returns a module.

Additionally, linting this module doesn't return any error. Can you provide a code or repo example which reproduces the error which this pull request fixes?

[PCManticore]

@brycepg This PR is for the "old" style namespace packages, not for PEP 420 (that one?) namespace packages. That is, it fixes the problem for the old pkg_resources hack where in __init__ one has to add __import__('pkg_resources').declare_namespace(__name__) to declare a namespace package.

I just tested this PR locally and it works great! Thanks @macbeth322 and super sorry for merging this so late!