Refactor NON_DATASET_PACKAGE approach to support packages

## Summary

In https://github.com/elastic/detection-rules/pull/3430 we spent time sanity checking additions to NON_DATASET_PACKAGE. We currently have to double [check several things](https://github.com/elastic/detection-rules/pull/3430#issuecomment-1946503428), indicating potential opportunities to refactor.

We may want to rely on the index pattern to parse this information. We should also create a checklist if needed for future PRs.

Start by reviewing PR 3430 and its linked comment to understand the checks currently required for NON_DATASET_PACKAGE additions. Trace where this approach is validated, then determine whether index patterns can replace those checks and document the resulting checklist; done means package handling is simpler and future additions have clear guidance.

Written by the indexing model from the issue text.