Internalize non-public API

[sync-by-unito]

Move non-public API out of public accessible namespaces. 

Don't know if this is possible with our current module approach, but maybe a good time to re-evaluate that then. 

An option could be to just more everything to internal packages, which is at least hinting that it shouldn't be used, but could maybe be backed by opt in requirements https://kotlinlang.org/docs/reference/opt-in-requirements.html

[cmelchior]

I suspect that we regardless need to move things to an io.realm.internal package, due to Java interop as everything with module visibility turns into public

[rorbech]

Just for reference. There seems to be some discussion on whether to support -Xfriend-paths across modules or maybe hide @RequiresOptIn annotations to make them unintentionally discoverable in https://kotlinlang.slack.com/archives/C0B8PUJGZ/p1603379870021800?thread_ts=1602247378.011300&cid=C0B8PUJGZ and linked posts/issues