The set of installed extension interfaces (in
`/usr/share/accountsservice/interfaces`) can change while
accountsservice is running, due to other packages being installed or
removed.

For example, it’s happened here:
https://github.com/flatpak/flatpak/issues/5264#issuecomment-2066667101



In order for the running state of the accountsservice daemon to match
what its state would be if it had been restarted after the directory
change, we need to monitor for changes and reload the set of extension
interfaces.

Do this in a straightforward way, by unregistering all of the extension
interfaces and re-registering the new set on any change. This shouldn’t
result in any visible churn on the bus because all the object
registrations are local. This approach might have to change if the
daemon were to start implementing `org.freedesktop.DBus.ObjectManager`,
because it emits signals on interface changes. Let’s cross that bridge
when we come to it though.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Fixes: #125