Previously @ServletComponentScan did not work when there was a
component index on the classpath as it made an assumption about
the concrete type of the BeanDefinitions produced by scanning that
does not hold true when an index is present.

This commit updates the scanning and the handlers to correct the
assumpution by working with a bean definition type that is produced
by scanning both when there is and when there is not an index present.

To prevent the problem from reoccuring, a test that uses and index
has been added and the import of ScannedGenericBeanDefinition is now
prohibited by Checkstyle.

Closes gh-18910