Add a new `@ImportAutoConfiguration` annotation that can be used by
tests that wish to selectively import certain auto-configuration
classes. Also add `@AutoConfigurationPackage` so that package
registration is decoupled from `@EnableAutoConfiguration`.

An added benefit of the change is @EnableAutoConfigurationImportSelector
can now be subclassed to provide custom annotation support if needed.

Fixes gh-3660
See gh-2772