Previously, when @EnableAutoConfiguration was used in multiple packages,
the last @EnableAutoConfiguration that was processed would
win and only its package would be stored as an auto-configuration
package.

This commit updates AutoConfigurationPackages to allow multiple package
name registrations. AutoConfigurationPackages.set(…) has been altered to
augment the constructor arguments of the BeanDefinition registered for
the initial call to the method so that the packages handed to the method
call will be added to the bean definition and not replace the previous
ones. The method has been renamed register(…) to reflect the changed
behavior.

Closes gh-1994