Pubdev: load authors from API instead of pubspec.yaml

The Pubdev loader currently uses intrinsic metadata (pubspec.yaml) to populate the revision author field.

The documentation discourages that: https://dart.dev/tools/pub/pubspec#authorauthors

However, it does not point to an API where we can get this info. We need to check how we can get it

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Migrated from T4628 (view on Phabricator)