packages_distributions doesn’t work for some dist-info packages built outside Setuptools

I assume that for those packages, something similar to the following will work to retrieve the top level packages/modules contained in a distribution

```py
{
    f.parts[0] if len(f.parts) > 1 else f.with_suffix('').name
    for f in pkg.files if f.suffix == ".py"
}
```

Of course there’s a lot of edgecases