Permalinks: look up from index if cache empty

[distantnative]

Description

@lukasbestle Implemented mostly as discussed in issue, though with Cache::isEmpty() instead of a special key to flag. However, I am very unsure if this really is a solution. Consider the scenario:

• Cache doesn't exist
• Permalink request now is allowed to use the index to resolve the UUID
• Finds the model which also populates the cache for this UUID
• Other permalink comes around
• Cache isn't empty anymore
• Returns false cause it won't find the UUID in cache and is not allowed to use index

So are we really solving the problem of the issue or in reality only postponing it one further request?

Summary of changes

• If the cache is empty, actually allow searching for UUID in index when resolving permalinks
• Cache::isEmpty() methods
• Cleaned up Cache classes (method order)

Reasoning

Additional context

Review commits one by one as the method reordering clutters the general diff.

Changelog

Fixes

Breaking changes

• Cache handler must implement ::isIndex(): bool method

Ready?

• In-code documentation (wherever needed)
• Unit tests for fixed bug/feature
• Tests and CI checks all pass

For review team

• Add lab and/or sandbox examples (wherever helpful)
• Add changes & docs to release notes draft in Notion