for "something else" there are 2 major options: pex_config (dislike because the 'data' gets executed), and yaml (very well supported, is just data). 3rd option: we could use the sqlite registry, and define a schema for that.

(sqlite benefits: has support for write locks that will be necessary very soon. Except, need write-once-compare-same mechanism; so we'll need that 1. for normal files and 2. across n nodes. so maybe there's no benefit).

also, while we're there:
as noted in DM-4170, registries.py needs to not use astropy and instead should use pyfits. If it's quick, that change should be made here (or turned into a separate ticket)