extensions: Unify how manager is injected into shared module

extensions: Unify how manager is injected into shared module

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>