Inconsistency with how libyosys is imported in Python

[donn]

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):

import libyosys as ys

or pyosys is used as a Python library:

from pyosys import libyosys as ys

Bringing those into alignment may be desirable.