ASYNC115 (async-zero-sleep): `anyio.lowlevel.checkpoint()` needs `import anyio.lowlevel`

### Summary

```python
import anyio

async def test() -> None:
    await anyio.sleep(0)
```

https://play.ruff.rs/86a8f945-5295-4092-ae43-81de88b90a63

ASYNC115 auto-fixes this to:

```python
import anyio

async def test() -> None:
    await anyio.lowlevel.checkpoint()
```

This doesn't type check because `anyio.lowlevel` isn't imported.

### Version

_No response_