deal with hashes in ftparchive more dynamic as well

deal with hashes in ftparchive more dynamic as well

Now that libapts acquire system happily passes around hashes and can be
made to support new ones without an ABI break in the future, we can
free ftparchive from all the deprecation warnings the last commit
introduced for it.

The goal here isn't to preserve ABI as we have none to keep here, but to
help avoiding introduction problems of 'new' hashes later as bugs creep
into the copy&paste parts, so short/less of them is good.