RawExtrinsicMetadata: Use ExtendedSWHID as target and remove type
ExtendedSWHID can identify either a software artifact or an origin, so we no longer need Union[SWHID, str].
Therefore, we no longer need the 'type' attribute, as it was only used to tell when the target is a SWHID and when it's an origin URL.
Note: this will break all packages handling values of RawExtrinsicMetadata.
Migrated from D5158 (view on Phabricator)