As detailed in
https://github.com/pytest-dev/pytest/issues/11475#issuecomment-1937043670,
currently with `--import-mode=importlib` pytest will try to import every
file by using a unique module name, regardless if that module could be
imported using the normal import mechanism without touching `sys.path`.
This has the consequence that non-test modules available in `sys.path`
(via other mechanism, such as being installed into a virtualenv,
PYTHONPATH, etc) would end up being imported as standalone modules,
instead of imported with their expected module names.
To illustrate:
```
.env/
lib/
site-packages/
anndata/
core.py
```
Given `anndata` is installed into the virtual environment, `python -c
"import anndata.core"` works, but pytest with `importlib` mode would
import that module as a standalone module named
`".env.lib.site-packages.anndata.core"`, because importlib module was
designed to import test files which are not reachable from `sys.path`,
but now it is clear that normal modules should be imported using the
standard mechanisms if possible.
Now `imporlib` mode will first try to import the module normally,
without changing `sys.path`, and if that fails it falls back to
importing the module as a standalone module.
This also makes `importlib` respect namespace packages.
This supersedes #11931.
Fix#11475Close#11931
Bruno Oliveira
c85fce39b6