[storage] Collections cleanup

[bkirwi]

Tease out some unnecessary code from StorageCollections.

• Remove the "status collection" field. This is no longer needed anywhere; we manage status collections fully in envd now.
• Remove the "data source" from our collection state. It's Weird that storage collections knew this level of detail in the first place, and there's a fair bit of code that's dedicated to keeping this metadata in sync. We grab the specific values that we care about and drop both the data_source field and all the code that keeps it in sync.

Motivation

It's very hard to make changes to storage collections! Simplifying makes it a bit easier to experiment with details of eg. how primary semantics work.

Breaking this out from my experiments since I think it's valuable cleanup in general and because I don't want it to bitrot as we get into the holidays.

Tips for reviewer

Very fine-grained commits, mostly for my own sanity - but it may help review as well.

[def-]

You'll want to rebase on main to get rid of the cargo test flake.

[aljoscha]

I like it, nice cleanup in there!

[aljoscha]

feels like we're taking a step backwards, with now adding a field that is specific for collections, while other parts of the PR remove specific knowledge about the collections we're managing

but also fine, I might now know all the context 🤷‍♂️

[bkirwi]

Totally. We unfortunately still need it to resolve the remap collection in create_collections_for_bootstrap, though, and without adding this field we wouldn't be able to remove the DataSource

I think the next incremental step is to change the storage controller to pre-resolve this info, so that storage collections doesn't need to know about the remap business at all, and can just track it as an opaque dependency. But that's followup work...