Harmonize and Simplify Shiboken Paths

The path settings are always quite complex in Shiboken.
In the build dir, we have shiboken6.shibokenmodule.shiboken6.so.
In the install dir, we have shiboken6.shiboken6.so.

• rename shiboken6.so to Shiboken.so
• unify the paths in `build` and `install`
• have one single shiboken import

  from shiboken6 import Shiboken
  

For compatibility, allow to also do this import

from shiboken6 import shiboken6

This should go away in version 6.2 .