Get rid of suppressions for "invisible_reference" and "invisible_member"

[psteiger]

For accessing internal declarations from "friend" modules (e.g. access rib-base internals from rib-test), we are currently suppressing compiler errors with @file:Suppress("invisible_reference", "invisible_member")

A better approach is to:

1. Create an internal opt-in annotation.
2. Make the "accessible to friend modules" component public
3. Mark the (now public) component with the opt-in annotation.
4. Opt-in to the annotation from build.gradle.

Because the new annotation is internal, it cannot be normally accessed from external modules. But Gradle can see it if it's part of the source set.

Benefits:

1. We get rid of the hacky suppressions in our codebase.
2. By suppressing invisible_reference and invisible_member we gain access to all internal components from the other modules, and that's not ideal: we'd rather be explicit about what are the APIs that are visible for module friends, and what are truly internal for the module and should never be used even from module friends.
3. We make the internal visibility of those friend-modules APIs even stricter, since consumers from the general public now cannot just suppress "invisible_reference" and "invisible_member" to directly access the friend-module API -- they require explicit Opt-In. They would need to suppress invisible members AND opt-in.

[psteiger]

Note: this is the same approach now being used in KotlinX libraries: Kotlin/kotlinx.serialization#2444