ExtensionUtils was originally used for shared functions between
the extension system and the (old) prefs-tool, but then gained
useful API meant for extensions themselves.
It's a bit weird to mix the two, so split out the extension convenience
API into a separate module.
We will soon split up the module further, and add specific "flavors"
for extensions and preferences, with the current code providing a
shared base for both.
That should explain both the new location and the odd module name. :-)
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2837>
