Fix: Prioritize searching for singletons in all class types when determining best creators

[justcarlux]

This addresses the following issue: #3663

For more information about this issue, please refer to my minimal reproducible example's README: https://github.com/justcarlux/protocollib-overriding-1.8-world-types-example

[dmulloy2]

Expanding this branch to every class makes StructureCache.newInstance() return any public static self-typed object, even when the class is mutable. Because StructureCache::newInstance is also the destination factory used by FieldCloner, deep cloning such a value can write into shared global state.

I think we can make this work for the most part by tieing it together with ImmutableDetector. I'll make a follow up commit and credit you as a co-author

[justcarlux]

Yeah, I knew that I might've had other side effects, I admit it was a rushed solution since I didn't take the time to read into the rest of the codebase properly. Thanks for looking into it though.