Make the fetcher cache more reliable and easier to reason upon

[thufschmitt]

Is your feature request related to a problem? Please describe.

The fetcher cache is a more-or-less generic untyped key-value cache. This makes it hard to reason about it, and might hide some tricky bugs.

Describe the solution you'd like

Give some structure, either to the cache itself or to the code that manipulates it

• Add some typing to the fields
• Add a set of helper methods to manipulate the cache

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

Priorities

Add 👍 to issues you find important.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2024-03-04-nix-team-meeting-minute-130/40830/1