Make <swh:add_to_origin> set Deposit.origin_url + disable aggressive deprecation warning
deposit.parent can be true in two cases:
- because we got <swh:add_to_origin> -> then the deprecation warning is raised before this check is reached, because 'deposit.origin_url' is set
- because we got a slug -> then we shouldn't raise this error (it breaks existing clients that didn't stop using <external_identifier> yet)