This allows to set CMAKE_INSTALL_PREFIX to something else than /usr and freecad as Python package is still installed into the correct directory.
For more details see: https://forum.freecadweb.org/viewtopic.php?f=4&t=67750
If INSTALL_TO_SITEPACKAGES is enabled, the freecad-namespace package (freecad/__init__.py) is installed in the python-sitepackage-dir. Further, the library install path (CMAKE_INSTALL_LIBDIR)
is used to find the FreeCAD shared library. If, for some reason, one wants to import another installed FreeCAD version it's possible to set the environment variable "PATH_TO_FREECAD_LIBDIR" to
point to the wanted FreeCAD-library (FreeCAD.so / FreeCAD.dll)
(this mostly aims at new modules and extension modules which want to use pip)
- any module having problems with nameclashes can use this syntax (from freecad import module)
- current imports still work (backward cobatibility)
- python extension moduels can be installed (pip) to python std path (eg.: site-packages)
- adding app, gui to the new freecad package:
```
from freecad import app
from freecad import gui
```
- syntax for importing c++ extension will not change
```
import FreeCAD as App
import FreeCADGui as Gui
```
