Separate concerns of @Configuration class selection from the need to
register certain infrastructure beans such as auto proxy creators.
Prior to this change, ImportSelector implementations were responsible
for both of these concerns, leading to awkwardness and duplication.
Also introduced in this change is ImportBeanDefinitionRegistrar and
two implementations, AutoProxyRegistrar and AspectJAutoProxyRegistrar.
See the refactored implementations of CachingConfigurationSelector,
TransactionManagementConfigurationSelector to see the former;
AspectJAutoProxyConfigurationSelector to see the latter.
ImportSelector and ImportBeanDefinitionRegistrar are both handled as
special-case arguments to the @Import annotation within
ConfigurationClassParser.
These refactorings are important because they ensure that Spring users
will be able to understand and extend existing @Enable* annotations
and their backing ImportSelector and @Configuration classes, as well
as create their own with a minimum of effort.
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