[BUG] import xxx.yyy as zzz broken with language_level=2 (regression from 0.29.x)

[da-woods]

Describe the bug

See code... Note that I compile with language_level 2

Code to reproduce the behaviour:

# testmod/cabc.pyx

import collections.abc as collections_abc
print(collections_abc)

Add an empty __init__.py to testmod (not sure if this is absolutely needed)

Compile with python3 cythonize.py -2if testmod/cabc.pyx && python3 -c "import testmod.cabc"

On the current master this print the collections module

Expected behaviour

It should print the collections.abc module (and it does on the 0.29 branch)

OS

Linux

Python version

>3

Cython version

current master

Additional context

Bisected to 4993ba6

[scoder]

This seems a critical bug that we should fix for beta-2. I expect quite a number of projects to keep their existing code base on language_level=2 at least in the beginning. It should be a viable migration path.

[da-woods]

Agree. I'm actually quite surprised that this has been missed until now, suggesting few people are using language_level=2 explicitly. But quite a few "bugs" on the fedora list looked like they'd be solved by language_level 2

[scoder]

Well, there is a test for this that runs in both language levels:
https://github.com/cython/cython/blob/master/tests/run/importas.pyx
What seems to be missing is the same thing from within a package. It's a bit tricky now that there are two implementations (__Pyx_Import() and __Pyx_ImportDottedModule()) which both seem needed for the LL2 case since it combines that Py2 preference for relative imports with an absolute import fallback.

I'm looking into it.