refactor: create core/ package init

[Blackgaurd]

Summary

• Creates `builders/server/core/init.py` to establish the `core` package namespace before moving modules into it in follow-up PRs.

[github-actions]

• main
  • refactor: create core/ package init 👈
    • refactor: move core server code into core/ directory #344
      • refactor: move core tests into tests/core/ directory #345