Update the usage of --slug or external_identifier in the deposit cli

During @douardda's tests of the deposit the only way to create an origin without a UUID was with a --slug flag. As described here: #2391 (closed)

Using the external_id from the metadata file if --slug isn't given is a better cli behavior.

I'm not sure it is feasible technically.

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Migrated from T2752 (view on Phabricator)