Refactor so that `sources` are mapped to `account_id`

[ramimac]

This is a major refactor, and breaking change. As a result, it is unlikely to be adopted in short order.

We could accomplish it in a backwards compatible way by creating an accounts_v2.yaml and a Github Action to programmatically generate accounts.yaml from the new source-of-truth.

Were we to make the change, the secondary goal would be to allow easy lookups based on either account_id or source for the other attribute.