must not use internals for public API

[jkowalleck]

follow-up of #1192
this is part 2 of the goal to prevent usage of internals.

goal

breaking: remove any usage of non-public classes in public API.
prepare a breaking change, but dont mere it yet; have it added to the next major milestone

[jkowalleck]

The non-breaking character of #1202 does not require further actions.

Anyway, this ticket is left open, in case something new comes up.