Inconsistency with how libyosys is imported in Python

### Feature Description

The method of importing `libyosys` in Python is different based on whether `yosys` is the primary process using a Python interpreter (such as for Python plugins and #4553):

```python
import libyosys as ys
```

or `pyosys` is used as a Python library:


```python
from pyosys import libyosys as ys
```

Bringing those into alignment may be desirable.
