Rename the `proto-model` module, and revise remaining root package names

[dpikhulya]

This PR:

• Renames the Chords Proto Model library to Chords Proto Values. The respective module is now named proto-values.
  • The root package for the library's declarations is now io.spine.chords.proto.value for Kotlin, and spine.chords.proto.value for Protobuf.
• Renames the Chords Protobuf library to Chords Proto.
  • The root package for its declarations is now: io.spine.chords.proto.
• Changes the root package for the Chords Core package to: io.spine.chords.core.