[Fix]: Clear mods by code data before (re)loading to prevent duplicate key errors

[JonathanFeenstra]

LoadAsync clears the ModViewModels and Mods collections, but not modsByCode which is also filled in this method.

In practice, LoadAsync currently only runs once when the EngineView is loaded, so the duplicate key errors don't actually occur in normal usage. However, if you're already clearing ModViewModels and Mods to prevent similar issues when it's called multiple times, you should clear modsByCode as well. Doing so could still be useful if you for example want to benchmark the method.