Break package tangle between o.s.aot and o.s.core

[snicoll]

Two hints registrar have been recently added that creates a cycle between those two packages. We should consider moving those, perhaps all the way up to spring-context as they aren't really useful without it.

[snicoll]

The tangle for spring factories hint is a little bit problematic as it relies on a package protected method in SpringFactories, cc @philwebb

[philwebb]

Tricky. There's just enough code in SpringFactoriesLoader.loadFactoriesResource to make copy/paste unappealing. We could perhaps make it a protected static method then have a private subclass in SpringFactoriesLoaderRuntimeHintsRegistrar to access it again. Either that, or just make that method public.

[philwebb]

It feels a bit odd to have spring-context add the factory hints, I think I'd prefer a org.springframework.aot.support package that takes care of registering hints for items in org.springframework.code.

It would also be really nice to have a checkstyle rule to enforce the layering.