Previously, if an auto-configuration class was (wrongly) located in a
candidate package for component scanning, the class was silently loaded
as an app configuration (i.e. with the wrong lifecycle).

This commit adds an `AutoConfigurationExcludeFilter` to
`@SpringBootApplication` so that such classes are automatically
filtered. Since they are registered in `spring.factories`, we can
silently ignore them since we know they'll be loaded later on.

Closes gh-7168