The Pipelines are grouped in Stages, like the CI jobs. Stages run in
sequence, one after the other, while the pipelines within a stage run in
parallel.

Each pipeline runs on an, individual, `BulkImports::PipelineWorker` job,
which enables:

- smaller/shorter jobs: Background jobs can be interrupted during
  deploys or other unexpected infrastructure/ops events. To make jobs
  more resilient it's desirable to have smaller jobs wherever possible.
- faster imports: Some pipelines can run in parallel, which reduces the
  total time of an import.
- (follow-up) network/rate limits handling: When a pipeline gets rate
  limitted, it can be schedule to retry after the rate limit timeout.
  https://gitlab.com/gitlab-org/gitlab/-/issues/262024