chore: cleanup WORKSPACE

This is a task related to eventual cleanup of WORKSPACE support. This will happen when the minimum supported version is bazel 9, that is the first version that removes the WORKSPACE support.

Q: when should that happen? I personally think that having that done when bazel 9 is released and a stable bazel version no longer has `WORKSPACE` is a good time.

The following is a list of random things that we would need to do:
- [ ] Remove entry_points in `whl_library`.
- [ ] `IS_BZLMOD_ENABLED` has to go.
- [ ] `TOOL_VERSIONS` has to probably go and we can migrate the config to `MODULE.bazel` instead.

This is a future cleanup tied to Bazel 9 and the removal of WORKSPACE support. Review the listed areas: entry_points in whl_library, IS_BZLMOD_ENABLED, TOOL_VERSIONS, and the proposed MODULE.bazel configuration. Done means resolving each checklist item once Bazel 9 is the minimum supported version.

索引モデルが issue の本文から書いたものです。