Consistent annotation names (#2140)

Add `Annotation` suffix to the private annotations to allow nicer code
using the constants.

For example one can use the current annotation names as a temporary
variable instead of unclear shortcut. Instead of this:

    rag := flagsFromAnnotation(c, f, requiredAsGroup)
    me := flagsFromAnnotation(c, f, mutuallyExclusive)
    or := flagsFromAnnotation(c, f, oneRequired)

We can use now:

    requiredAsGrop := flagsFromAnnotation(c, f, requiredAsGroupAnnotation)
    mutuallyExclusive := flagsFromAnnotation(c, f, mutuallyExclusiveAnnotation)
    oneRequired := flagsFromAnnotation(c, f, oneRequiredAnnotation)

Example taken from #2105.