fibermap location in sims

Originally ICS was to provide the fibermap, but instead it provides a suite of files that have most of the needed information, from which we create our own fibermaps using `assemble_fibermap`, putting the results into the redux preproc/ directory. Sims haven't caught up with that and still expect the fibermap to be in the raw data directory, which creates a bit of a mess for `desispec.io.findfile`. Figure out what to do about that and clean it up.