Generate repository indexes

Generate repository index files for debian:suite collections. This is the first step in adding repository hosting to Debusine.

From our planning document:

MUST: Generate indexes for a repository

• Define the artifact categories used to store repository indexes
• Define relations between index artifacts, if any
• Define how overrides are stored and provided
• Enable debian:suite collection to store such artifacts as well as packages
• Implement the logic to actually generate the indexes
  • Likely as a server task processing a collection into a set of artifacts
  • Integrate overrides and repository metadata
  • Implement in an extendable way so that it’s easy to later add AppStream metadata, Translations, or other possible repository metadata
  • Scheduling (triggered, cron, …)
  • Contents, Packages, Release might need to be separate tasks in a workflow; Release could be generated in a lock-free way by considering the state of the suite at a given timestamp