Resource loader: mark impl as internal, fix deprecated loader API use

[apple502j]

This marks the rest of impl classes as internal, marks ResourceManagerHelper as non-extendable, fixes use of deprecated Loader APIs, and fixes javadocs.

Originally included changes on keyed reloaders (support for predicate/modifier) but excluded in this PR. In v1 we should make this use event phases.

[Juuxel]

Should the internal annotation be on the entire package instead of the individual classes?

If a package is marked with this annotation, all its containing classes are considered internal.

https://javadoc.io/doc/org.jetbrains/annotations/latest/org/jetbrains/annotations/ApiStatus.Internal.html

[apple502j]

Ah, that could be better. Maybe I should use that instead.

[Technici4n]

@apple502j Mind fixing the build? :)

[apple502j]

This can be merged to 1.19.2, seems like the 22w42a port refactors this completely making this unnecessary.