fix: autohooks prefix concatenation

[climba03003]

Since no core member add value to https://github.com/fastify/fastify-autoload/issues/376#issuecomment-2129429693 I want to move things forward to users desire behavior.

Fixes #376

As mention in last submitted PR, I believe the long term solution is refactor the code to create a single tree that contain both hook and plugin information.

Checklist

• [ ] run npm run test and npm run benchmark
• [ ] tests and/or benchmarks are included
• [ ] documentation is changed or added
• [ ] commit message and code follows the Developer's Certification of Origin and the Code of conduct

[climba03003]

Changing of test seems to indicate this is breaking, but actually not.

The new behavior is introduced in 5.8.1 and it was never tested before. And that is the one introducing new breaking behavior (or called non-clarified behavior).

Here is just reverting to behavior to the user most expected one.