Change holdings_id_map to only create mapping entries based on "legacyIdentifier" rather than "formerIds"

As of v1.7.1, the holdings transformer tasks populate the holdings_id_map based on the list of formerIds rather than the legacyIdentifier. For holdings, this can lead to unintended collisions between records and prevents using "salted" identifiers to create more than one holdings for the same legacy identifier (eg. Sierra checkin records with more than one location). I'd like to go ahead and change the map creation to only use legacyIdentifer.