Decouple loose indexes

[peterwaller-arm]

Depends on #29, so beware full diff contains that diff.

This builds on #29 (remove repository index).

• It removes the notion of a PackId::Loose. I intend to go a bit further here and get rid of PackId as an enum but that can be done later in a no-functional-changes PR.
• It removes --pack|-P from the command line interface, simplifying the user interface a bit.
  • There is a new API, repo.find_snapshot(maybe_canonical_snapshot) -> SnapshotId. This API returns an error if the given string does not uniquely identify a snapshot within a single index.
  • Users can specify a pack with the syntax pack:snapshot, or just snapshot, which feeds through this API.
  • Use of : as a packid:snapshot_name separator enables / to be used for namespacing packs by using directories on the filesystem, and for / to appear in snapshot in the future too, if we wanted.
• Store is reimplemented. It now no longer has to do work which scales with the number of snapshots which makes it quite a bit cheaper (it was taking 10s of seconds per snapshot when you have lots of snapshots, but now it's a flat 100ms).
• Stores are now independent so we can execute multiple of them in parallel sharing the object store safely.
• Stores save the snapshot to an index called packs/loose/<snapshot_name>.idx. So a loose snapshot is uniquely identified with loose/snapshot_name:snapshot_name, for the time being, if snapshot_name exists in other packs, or just with the string snapshot_name otherwise.
• Packing is generalized to take multiple indexes as input. If the indexes are not specified, all loose indexes are taken.
  • TODO: Check that I remembered to sort indexes by name.
  • Including an existing pack during pack creation is not yet supported, but I have a route in mind to implement this: Support including an existing pack during pack creation #35.
• Introduce a 'loose' bit to the PackIndex #34 For now loose packs are identified as index files lacking a pack. I intend to introduce a bit to the beginning of an index file identifying it as loose, so we can have 'indices' without having the packs.
• After this PR, loose indices are not removed during pack creation, nor are loose objects cleaned. Implement 'elfshaker gc' to clear unneeded loose objects and packs #17 is therefore a little more important. And probably we want to delete the loose indices by default.

There is one slightly confusing nomenclature issue with this PR, in that we refer to loose packs as packs, when they aren't yet packed. So the loose snapshots live in a 'loose/name.pack.idx'.

Resolves #15.

[peterwaller-arm]

I'm going to pull out the fs2 / locks changes into a separate PR. Other than that, this is ready to go.

[peterwaller-arm]

OK, I'm now done with this. Left atomic writes alone here, got rid of the fs2 dependency, which can be thought about separately as part of #40.

[peterwaller-arm]

Assuming it's green, I'm done fiddling here. The last action broke the tests again.

[veselink1]

Great! There will be no slowdown now when loose snapshots pile up.
A couple of comments, some might be non-trivial to resolve, sorry 🤔.

[veselink1]

Great, everything's fixed and we're now sorting by mtime, which preserves the order of the snapshots, thanks!

Typo in check.sh causes the tests to fail.

[peterwaller-arm]

Frustratingly elfsshaker store twice in quick succession is resulting in identical nanosecond-granularity timestamps in the mtime field, causing the test to non-deterministically fail. So I chose an EPOCH=$(date) and manually set distinct timestamps on the packs.