Debug flag to log out all mods without dependents

[sisby-folk]

Hey hi! here's a pretty obscure and simple debug feature.

In modpack dev, it'd be useful to have a way for QL to log out a list of all mods that have no dependents.

The reason this is useful is that in larger packs, if a library exists on this list - it can be removed. This is suprisingly easy to have happen accidentally in modpack development, and it's not entirely reasonable to try and resolve which libraries are unused manually.