debian:suite collection should include artifacts for index files

The debian:suite design doesn't currently include artifacts for the index files (InRelease, Packages, Sources, etc.). This seems like an oversight. Presumably we need new artifact categories for each kind of index file that can be added to the debian:suite collection.

We need to work out how this should work for external suites (index files mirrored every time the APTMirror task is executed) vs. suites managed by debusine (not currently planned, but eventually it would be nice if debusine could generate suitable index files itself). For the time being it seems fairly clear that index files should be optional.

Some repositories keep older versions of lower-level index files around in by-hash directories to improve mirroring robustness and support a limited form of snapshotting. https://wiki.debian.org/DebianRepository/Format has more details of this. The design should support that.

Although I thought of this while working on #331 (closed), I don't think that issue needs to block on this one: we can implement the APTMirror task without storing copies of the index files. It will be a bit less efficient than it might be, because it will have to download all the relevant index files each time it runs before it can tell whether it needs to update the collection. (Perhaps it could cache them somewhere, but I'm not inclined to bother since any such caching code would only be a stopgap for artifact storage.)