deposit_read: Simplify api to return only relevant deposit information

With this, the deposit information needed for the deposit loader to build the revision are sent in the "deposit" key. It contains the following keys:

• id: Deposit id
• collection: collection name
• client: client name
• author_date: creation date
• author: author
• committer_date: publisher date
• committer: publisher

This avoids:

• unnecessary redundancy. Metadata were set in multiple places even though identical.
• some information were no longer used (branch name, synthetic, revision type, etc...)

Test Plan

tox

Migrated from D3047 (view on Phabricator)