ModuleNotFoundError with import-mode=importlib and src layout

[jaraco]

In #3396, I'm exploring a fix for namespace packages, but the fix requires the use of import-mode=importlib. When I enable that setting on configparser, it fails with:

ModuleNotFoundError: No module named 'src.backports'; 'src' is not a package

I tried applying the fix in #7936, which promises to address the issue, but the issue persists.

In this case, the failure is not in the tests, but in the package itself. backports.configparser should be able to import relative to that package.