Change internal classes to internal

[304NotModified]

Proposed for 6.0.

From #551:

• Make the ReflectionExtensions helper internal and remove OrNull suffix as we use nullability for that.
• Do not expose internal classes from Core namespace, but instead expose interfaces.

[Romfos]

Part 1 is here: #869

Some core types are still public because they are used in public interfaces and change for them could be a bigger breaking change then proposed in this PR.

but in any case we can create part 2 with refactoring for other types

@304NotModified could you please investigate this?