`load_before` should allow setting the same mod as `optional_dependency` or `dependency`

Right now, `load_before` is equivalent to an `optional_dependency`, but in most cases, mods using `load_before` will depend on the mods in `load_before`. If the mod is loaded without the mod in `load_before`, it will most likely crash the game.

So, two options:

🎉: Just add the dependency check for `load_before`, i.e. don't load it if the dependency is missing.
🚀: Add the option to set the `load_before` mod also in `optional_dependency` and `dependency`. This is probably not as nice because we need to add additional checks in the existing dependency handling, as usually the dependency should load before the dependent mod.