Previously, only root auto-configuration classes could be excluded
eagerly via an AutoConfigurationImportFilter. Any configuration class
loaded as a result of processing a particular auto-configuration were
parsed and checked as usual.

This commit makes use of the `getExclusionFilter` callback to expand
this filter to all candidates that are considered. The annotation
processor has also be expanded to generate metadata for non-root
configuration classes.

Closes gh-12157