Unable to use legion macros unless legion is a direct dependency

Legion is re-exported through a library I'm using, however I can't use the `#[system]` and `#[resource]` macros without having a direct dependency on legion in my project.

The simple solution for users, that I think is fine, is to just add a direct dependency on legion if you want these features. Just thought I would bring this to attention in case this isn't ideal to the project maintainers.