Is your feature request related to a problem?

We should remove the innermost index.json to prepare for the 1.0 release of the GraalVM Reachability Metadata repository.

Describe the solution you'd like

As buildtools doesn't seem to use this index.json, we only need to check where it's used within this repository and refactor it so we don't need to use it (probably by parsing files/paths instead).

Describe alternatives you've considered

No response

Additional context

No response