pyside does not allow to override PYTHON_SITE_PACKAGES like shiboken does

shiboken allows to override PYTHON_SITE_PACKAGES, but pyside does not

which is required to allow install on conda where the site packages dir is not a subdirectory of the prefix

see sources/shiboken6/cmake/ShibokenSetup.cmake:

if(NOT PYTHON_SITE_PACKAGES) 
    shiboken_internal_set_python_site_packages() 
endif()

whereas sources/pyside6/cmake/PySideSetup.cmake has:

shiboken_internal_set_python_site_packages()

spdir.patch

^{cc ctismer}