Make use of bean definition overriding more visible

[snicoll]

With errors that are hard to track, and with XML bean definitions taking always precedence over Java config (which can be surprising), we'd like to deprecate bean overriding wholesale with the idea of removing the feature at some point.

Spring Boot has already paved the way by disabling it by default and introducing a property to opt-in to the previous behavior. Us deprecating the feature in the core framework should send a strong signal to developers that they should not rely on that feature anymore.

[dearoneesama]

It scares me off, but It looks like we are not talking about overriding @Bean-decorataed methods right?

[snicoll]

registering twice a bean definition for the same key (i.e. bean name).