cad/opencascade: switch to vtk9
- switches opencascade to vtk9 to enable upcoming import of cad/py-ocp
- cad/freecad has to switch vtk8 -> vtk9, too
- this requires upstream commit 0cfea3fee3e7848bbf043d2b1a19f6405d7ebe25 "Make smesh compile with vtk9"
- while touching this, fixes vtk module detection
- clean up VTK_DIR usage: that variable does not exist in FreeCAD's build system anymore (for quite some time, actually)
Obtained from: opencascade upstream: Kirill Gavrilov
Obtained from: freecad upstream: committed by github/wwmayer
Differential Revision: D30934
Reported by: thierry@
Submitted by: thierry@