Check excluded auto-configuration classes are actually auto-config

[philwebb]

See #6861 for one example where this would help. We could check that if the class exists, it should be in the list that would be otherwise included.

[snicoll]

@AutoconfigureAfter/Before is another candidate. I've seen a few users trying to use that thing on regular configuration classes.

[wilkinsona]

I've merged and closed #7407 but left this open so that we can look at doing something similar for @AutoConfigureBefore and @AutoConfigureAfter

[mbhave]

@philwebb For the @AutoConfigureBefore and @AutoConfigureAfter, I see two ways in which it could be incorrectly used:

1. The annotation is used on a class that is not auto-config
2. The annotation value contains a class that exists but is not auto-config.

Point 2. could be taken care of in the AutoConfigurationSorter but I'm not sure if we also want to fail hard on Point 1. Thoughts?

[philwebb]

Closing this one as fixed. We'll take on AutoConfigureBefore/AutoConfigureAfter in #7697 which we might slip.