Catmandu::Fix::external_id - transform external_id array into a hash
# Transform an array of external_ids into a uniq list of hashes
# external_id:
# - type: opac
# value: 12345
# - type: opac
# value: 6789 (copy)
# - type: arxiv
# value: 0001
external_id()
# external_id:
# opac: 12345
# arxiv: 0001