Fan out backfill tasks

[di]

Follow on to #15368: In order to avoid deadlock, this PR introduces a one-to-many pair of backfill tasks that fan out tasks to individual files, which are modified in a per-file transaction.

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ KeyError: 'backfill_metadata.batch_size' warehouse.packaging.tasks.backfill_metadata View Issue
• ‼️ TooManyRequests: Too many requests warehouse.packaging.tasks.metadata_backfill_ind... View Issue
• ‼️ ServiceError: 500 internal_error incident id a575f26b900b-f359f54e9c648a31 warehouse.packaging.tasks.metadata_backfill_ind... View Issue
• ‼️ OSError: [Errno 36] File name too long: '/tmp/tmp64hec5o5/uselesscapitalquiz-3.141592653589793238462643383... warehouse.packaging.tasks.metadata_backfill_ind... View Issue
• ‼️ KeyError: PosixPath('METADATA') warehouse.packaging.tasks.metadata_backfill_ind... View Issue

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