Safe, dependency aware merge train car re-ordering

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Proposal

Merge trains with predictive pipelines, where runtime depends on the actual changes, can benefit from train car re-ordering. It would be great if we could enable this without compromising on safety, and without having to rebuild the entire train. To this end:

  1. Review Uber's submit queue blog post and similar work for correctness and viability.
  2. Assuming it all checks out, implement dependency awareness for pipelines, and leverage it for merge trains. Since it is theoretically impossible for us to implement omnipotent dependency awareness, this will likely come come in the form of a artifacts:report.
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