Move runtime classes under `io.spine.v8n.runtime` package

[alexander-yevsyukov]

Currently, we have two parts of code that are required on runtime in the generated code:

• The io.spine.validate package, which comes from v1 version.
• The io.spine.validation.api package introduced in the v2 version.

We need to have the one package or at least one root package for runtime classes. The word validate was selected because of its shortness, and after the package rom Java Validation.

Since we're migrating towards using more Kotlin, and our library is too far from Java Validation already, we don't have to stick with the old names.

The suggested package name is v8n which is short, and comes in the line with i18n and l10n acronyms.

It is still to be decided if code generation components would go under io.spine.v8n.compiler package. But such a move looks logical in the light of other renaming works that are currently in progress.

[alexander-yevsyukov]

@armiol, FYI.