Resolvesspring-cloud/spring-cloud-stream#1083
By default Spring Framework allows beans overriding via the same name.
The binding target definitions (`@Input` and `@Output`) populate beans as well
and when we use the same name for target we end up with unexpected behavior
but without errors.
Since it isn't so obvious via Spring Framework bean definition DSLs
(XML or Java & Annotations) how to override beans with the same name,
that is absolutely easy to use the same value for `@Input` and `@Output`
definitions even in different binding interfaces.
That's hard to analyze fro the target application since mostly
`@Input` and `@Output` produce `MessageChannel` beans.
* Fail fast with the `BeanDefinitionStoreException` when we meet existing
bean definition for the same name
* Add JavaDocs to the `@Input` and `@Output` to explain that their `value`
is a bean name, as well as destination by default
Since `@EnableBinding` is `@Inherited`, the inheritor picks up it from the
super class during configuration class parsing.
The parsing process logic is such that after the root class we go to parse its
super classes, and therefore come back to the `@EnableBinding` again.
In this case we process all the `@Import`s one more time and collect them to
the root `configurationClass`.
Essentially we get a duplication for the `ImportBeanDefinitionRegistrar`s
such as `BindingBeansRegistrar`.
The last one parsed `@EnableBinding` and registers appropriate beans for the
`@Input` and `@Output`, as well as for the binding interface - `BindableProxyFactory`.
But since we have it twice in the `configurationClass` we end up with
`BeanDefinitionStoreException` mentioned before.
That's how Spring Framework works with inheritance for configuration classes
and that's may be why it allows to override beans by default
* Skip parsing `@EnableBinding` one more time if the bean definition for
binding interface is already present in the `registry`
* Fix `AggregateWithMainTest` do not process `@ComponentScan` what causes
picking up the configuration classes for children contexts in the aggregation
* Fix `testBindableProxyFactoryCaching()` do not register `Source` and `Processor`
in the same application context because both of them cause registration for the
`Source.OUTPUT` bean
4c33e5eb58