Don't delete METS before indexAIP

indexAIP requires the AIP's METS to exist, but ordinarily it is
deleted after compression. indexAIP handles this by extracting the
file from the compressed AIP, which seems wasteful.

Instead, skip deleting the AIP METS in the first place, and use that.

refs #7424
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