`NotRegisteredBeansReconciler` doesn't take factory files into account anymore

[martinlippert]

Due to the switch from looking up symbols to looking up beans in the index to verify whether certain types have been registered as beans, this reconciler does not take beans defined in *.factories files into account anymore.

The fix should include:

• test case that runs the reconciler on a real project (in addition to the existing reconciler test that purely tests the logic of the validation itself)
• test case improvements for the factories indexer to verity that index elements are created, not only symbols
• fix for the factories indexer to create index elements

Since this creates a dependency between a validation on Java source code and content inside of those *.factories files, we probably also need to think about the mechanism that keeps track of those dependencies, the exceptions to signal a complete index, etc. But that is probably enough work for a separate issue then.