Simplify BeanRegistrationsAotProcessor to avoid unnecessary BeanRegistrationKey

[snicoll]

BeanRegistrationsAotProcessor has a map of registration key to registration but the mapping does not seem to be used anywhere. It looks like the Registration does not directly expose the RegisteredBean (which would provide the bean name and the class that the key holds) but we very well could.

This would give a list of Registration where each Registration has the RegisteredBean to process. Internally, it's not possible to register the same bean twice since we loop over bean names.

This was uncovered as part as reviewing the exception strategy.