Fix import issues with Python GUI modules
This adds 'vtkmodules' to the vtkRenderingTk search path (this was missed when vtkmodules was introduced in VTK 8.2)
It also imports vtkRenderingOpenGL2 and vtkInteractionStyle into the examples associated with the Python GUI modules. These are the minimum imports needed for these example, unneeded implementation modules such as vtkRenderingVolumeOpenGL, vtkRenderingFreeType, or vtkRenderingContextOpenGL2 are not imported.
Closes #18350 (closed)