Do not generate unused components contents in
@kaspth we talked about it.. I think there might be an unmerged PR?
IIRC I wasn't excited about adding an extra top-level file for such a rare consideration (plus the potential confusion if you've added/removed a component since the initial generate), so this does seem potentially interesting to me.
Currently, `app:update` generates all contents regardless of the component using in application. For example, even if not using Action Cable, `app:update` will generate a contents related to Action Cable. This is a little inconvenient. This PR checks the existence of the component and does not generate unnecessary contents. Can not check all options in this way. However, it will be able to prevent the generation of unnecessary files.