Validation: Don't allow the same mod id in `load_before` and `dependencies`

As mentioned in 
- #185

> Mods added to load_before should not be added to dependencies this cancels out the effect.
Because a cyclic reference is created and the dependency check will break out of the loop, so both mods end up with the same importance score.

To avoid this we can add this as a validation check for the manifest.