Werk #16561: Fix GUI extensions with CEE specific files break CRE

In previous 2.3 beta releases the UI of the Raw edition could break in case extension packages were installed which contained files specific to the CEE, like e.g. agent bakery plugins.

An exception like this was displayed in that case:

Traceback (most recent call last):
  File "/omd/sites/cre/bin/cmk-update-config", line 8, in <module>
    from cmk.update_config.main import ensure_site_is_stopped, main
  File "/omd/sites/cre/lib/python3/cmk/update_config/main.py", line 36, in <module>
    from cmk.gui import main_modules
  File "/omd/sites/cre/lib/python3/cmk/gui/main_modules.py", line 44, in <module>
    import cmk.gui.cee.registration  # pylint: disable=no-name-in-module,cmk-module-layer-violation
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ModuleNotFoundError: No module named 'cmk.gui.cee.registration'

To the list of all Werks