Migration-utils: Additional configurability with storage path layouts

Layout. In flat and pairtree migrations the PID is used to form the path; for example PID nlm:nlmuid-101588995-bk (stored FOXML file name nlm_nlmuid-101588995-bk) becomes /ocfl/nl/m+/nl/mu/id/-1/01/58/89/95/-b/k/5-bk. Characters such as – are problematic in Linux.
It would be nice to declare use of another field, or input map, to dictate the value to use for layout path generation. For example, it may be nice to use 101588995_bk to generate a path for PID nlm:nlmuid-101588995-bk.