as_PIL option broken in extract_frames_as_images due to _check_cv_dep import behavior

### Bug Description

`extract_frames_as_images(..., as_PIL=True)` fails with `AttributeError: module 'PIL' has no attribute 'Image'`.

### Cause

`_check_cv_dep()` (added in 1.3.0) uses `importlib.__import__(dep)`, which returns the top-level package, not the requested submodule. So `_check_cv_dep('PIL')` returns the `PIL` namespace package without loading `PIL.Image`.