upgrade: register all format variants in a list Now that all known format variants exists outside of the function, we can gather them in a lists. This build a single entry point other code can use (current target: extensions). The repository upgrade code is updated to simply use entries from this list. As a side effect this will also allow extensions to register their own format variants, to do this "properly" we should introduce a "registrar" for this category of object. However I prefer to keep this series simple, and that will be adventure for future time.