Hide unintentionally public API

[qurbonzoda]

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Both the :plugin and :runtime modules have many APIs that were accidentally introduced with a public visibility modifier. These APIs are currently annotated with @KotlinxBenchmarkPluginInternalApi and @KotlinxBenchmarkRuntimeInternalApi in the plugin and runtime, respectively.

The annotations require opt-in with a WARNING level. This level should be advanced to ERROR in a future release, and eventually, the annotated APIs should become internal, ceasing to be part of the library's public API.

Once the APIs are made internal, make sure to drop those that are not used internally.