F401 unused import false negative

False negative
```
import importlib # this package is unused, but not detected
import importlib.util

package = "ruff"
is_present = importlib.util.find_spec(package)
```
When `from importlib import util` is used, correct behaviour is observed
```
import importlib # this package is unused, and detected
from importlib import util

package = "ruff"
is_present = util.find_spec(package)
```

Ruff 'v0.0.270'

(hope I'm overlooking some submodule complexity in the general case, but it seems that in this specific case `import importlib` can be safely removed)

Start with the two Python reproductions in the issue and compare their F401 results under Ruff. Trace the import-analysis entry point for F401 and add or update coverage for the importlib/importlib.util case; done means the unused importlib is detected without changing the correct from-import behavior.

Written by the indexing model from the issue text.