lib.mkRemovedOptionModule: Show replacement for option usage too

Previously mkRemovedOptionModule would only show the replacement
instructions when the removed option was *defined*. With this change, it
also does so when an option is *used*.

This is essential for options that are only intended to be used such as
`security.acme.directory`, whose replacement instructions would never
trigger without this change because almost everybody only uses the
option and isn't defining it.