pack-generation wouldn't work with alternate DBs

[Byron]

When finding an entry by location (as used by pack generation code) it will compare by pack_id.

Earlier it was a hash over the path of the pack, but recently became just an index of an iteration in order to prevent any possibility of clash.

Re-introducing the hash could work, but it's probably better to remove the idea of alternates as a bunch of compound DBs entirely and see repositories as a list of paths with objects to build a combined view of objects there.

Related to #259