Remove repository index

[peterwaller-arm]

Draft.

We plan to remove the RepositoryIndex for now and make it fast enough to
consult the packs directly.

This removes the notion of update-index from the UI, and gives us a single source of truth: the pack indices themselves.

Depends: #27, #28 (only look at last commit).

[veselink1]

Looks good.
I'm thinking we might want to cache the indexes at some point? We will end up reading the same index several times otherwise.

[peterwaller-arm]

I'm thinking we might want to cache the indexes at some point? We will end up reading the same index several times otherwise.

Do you mean cache them in memory? If so, is there a specific operation you had in mind? I was thinking that if some operation needs repeated access to an index it should just retain a loaded PackIndex.

[veselink1]

Cache in memory, yes. Resolving a snapshot name opens and reads all indexes. Opening the pack to extract opens and reads the pack index again.
If we allow repacking at a later stage, and we will probably allow the user to specify multiple snapshots to pack, each one will need to be resolved separately.

[peterwaller-arm]

Resolving a snapshot name opens and reads all indexes

At the moment yes, but in the future this will just involve consulting the list of snapshots at the front of the pack. I'm expecting this to be efficient enough to be able to do without the need for a cache.

If we allow repacking at a later stage, and we will probably allow the user to specify multiple snapshots to pack, each one will need to be resolved separately.

I think in this case what we coud do is first resolve the total set of packs requested, then consider each pack once, resolving requested snapshots as we go (in the order they appear in their indices). Then reorder those resolved snapshots back to what the user requested.

[veselink1]

I forgot that we're planning to pull the snapshot tags at the front. We'll have to see how that plays out in practice I guess.