Promote kimchi_backend_common and kimchi_pasta

[georgeee]

Promote these two sublibraries to be packages of their own. This allows to remove three-layered sublibrary names.

More importantly, it removes some package-to-package circular dependencies and overall makes things a bit cleaner.

Clarification

It's package-to-package circular dependency, not libray-to-library. Dune handles it well. But dependency diagram becomes way less readable, also it makes packages incompatible with findlib-based installation approach (because one can't install packages one by one, they depend on one other).

Explain how you tested your changes:

• Mina compiles

Checklist:

• [x] Dependency versions are unchanged
  • Notify Velocity team if dependencies must change in CI
• [x] Modified the current draft of release notes with details on what is completed or incomplete within this project
• [x] Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• [x] Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• [x] All tests pass (CI will check this if you didn't)
• [x] Serialized types are in stable-versioned modules
• [x] Does this close issues? None

[georgeee]

!ci-build-me

[georgeee]

!ci-build-me