We unified most code paths earlier, but the common code will still
import Main locally if no extension manager was injected before.
Now that the old extensionUtils was split between extension and
preferences, each of those modules can simply import the manager
from its corresponding environment, and then inject it into the
shared module.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2837>
For the time being this mostly means re-exporting functions
from the shared module. However openPrefs() is now only
available to extensions, and we stop exporting both
getCurrentExtension() and setExtensionManager() to either
extensions or prefs.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2837>
