Avoid falsely marking non-submodules as submodule aliases (#6182)
## Summary
We have some code to ensure that if an aliased import is used, any
submodules should be marked as used too. This comment says it best:
```rust
// If the name of a submodule import is the same as an alias of another import, and the
// alias is used, then the submodule import should be marked as used too.
//
// For example, mark `pyarrow.csv` as used in:
//
// ```python
// import pyarrow as pa
// import pyarrow.csv
// print(pa.csv.read_csv("test.csv"))
// ```
```
However, it looks like when we go to look up `pyarrow` (of `import
pyarrow as pa`), we aren't checking to ensure the resolved binding is
_actually_ an import. This was causing us to attribute `print(rm.ANY)`
to `def requests_mock` here:
```python
import requests_mock as rm
def requests_mock(requests_mock: rm.Mocker):
print(rm.ANY)
```
Closes https://github.com/astral-sh/ruff/issues/6180.
