Implement whatever search features the tape archiver needs

[pkgw]

The on-site tape archiver will need to ask the Librarian for a list of files that it cares about that do not have instances on the magic "tape" store (cc #30) used for files that have been written to tape.

It should also be able to get a list of results including the file sizes and, ideally, the path to a file instance that it can access in order to copy files over to a staging .tar file for writing to the tape.

CC @plaplant