indexAIP: index MODS identifiers

This extracts identifiers from MODS documents contained within an AIP's
submissionDocumentation, and indexes them in the search index.

A new key has been added to the ES JSON document: 'identifiers'. This
key contains an array of zero or more strings, which are intended to
be identifiers from other systems cataloguing this data. For example,
in a transfer from Islandora, the identifiers might represent the
original identifiers of items in Islandora.

These MODS documents are looked up within the
submissionDocumentation/mods directories from transfers within the AIP;
they should be present within the submissionDocumentation/mods
directory in the transfer at the time that the transfer begins.

refs #7424