Save full list of entries for every source

When we normalize sources by deduplicating on `sourceId`, we end up losing a little bit of data that's important for being able to reconnect `source` objects with their lines in `library/foo`.

This resolves that by adjusting our `entry` object to be a vector of `entries` instead, but sorted using the same `SOURCE_DATE_EPOCH` tiebreaker we used previously so that `.entries[0]` is the same as our old `.entry`, but we keep the full list of values for later use/lookup/cross-referencing.

I have additionally verified that `meta.jq` here was the only place we are actively consuming from the `.entry` object (currently), so this should be fully sufficient (no changes necessary elsewhere).

Author: tianon-sso