Add a warning when user code refers to `scala.runtime.*`

[Gedochao]

Extracted out of #23804

There are no guarantees for user code relying on API in scala.runtime.*, but we currently do not warn about it at all.

Thus, regressions like #23804 may occur, i.e. when referring to scala.runtime.stdLibPatches.Predef.assert

Expectations
Annotate scala.runtime.* as internal API and print warnings when they're used.

[Gedochao]

We may want to consider doing this in 3.8 😕
cc @hamzaremmal @sjrd @natsukagami

[He-Pin]

In Akka there are @internalapi @InternalStableApi for this.

[som-snytt]

How many times have I reached for scala.runtime.ScalaRunTime.stringOf? Even though it could have been improved.

Perhaps there are three approaches:

• package deprecation
• build tool plugin to enforce do-not-call list
• a -release mechanism to enforce what API is available (from the Scala platform and/or JVM platform)

All of these have been requested.

I think people want to use stringOf or println without getting hassled, but to fail C.I. if they accidentally forget to remove debug statements.

[soronpo]

Why not change it to protected[scala]

[He-Pin]

@soronpo A user can still reference to it if by creating an scala package and put code there.

[soronpo]

@soronpo A user can still reference to it if by creating an scala package and put code there.

That's perfectly fine, IMO. They take the risk doing so.