Make some native vectorization classes package private to be consistent and a minor fix

[shubhamvishu]

Description

• Addresses some comments on #15508 to make some classes package private for consistency and a minor fix to method name (we'll eventually planning to remove it in Add/Improve the test coverage for NativeVectorUtilSupport #15840).

[uschindler]

The slow code is at least working. It is a not of work.

For testing we can use a trick without native code: Maybe define java methods with correct signature with scalar impl and hook them in for testing. So method able to be called is just a mich deleting to a java class.

[uschindler]

I still think this should be different. Keep the invokeExact() here and convert the MethodHandle for foreign function using asSpreader(Object[].class, numberOfParams). That's a bit faster, but still has the varargs array creation and conversion of primitives to wrappers.

[shubhamvishu]

Make sense, I switched it to back this invokeExact way

[uschindler]

This may not work so easy, so let's do that later. We need tests. But in the ideal case we should call all methods statically. Its more try-catch logic but it is needed for performance.