Allow arbitrary packages to be scanned for Reflective usage

[snicoll]

Follow-up of #29194

We'd like to make sure that non beans can define @Reflective usage (or any composed annotations using it), so that any type can provide that information. It's a bit unclear how we can integrate such scanning. I don't know if we should go as far as introducing a @ReflectiveScan that behaves the same way as @ComponentScan but for reflective usage.

I already have a working implementation that needs to be called explicitly with the packages to scan, see snicoll@24dc359.

Whether or not we want to introduce @ReflectiveScan is largely dependent on how Spring Boot would like to integrate this feature. Ping @wilkinsona for feedback.

[snicoll]

As part of fixing this, we should make sure to review the javadoc of the existing annotation as well as the reference guide to provide examples to users.