Separate AnnotationVerifier from BestPracticesVerifier

This is just a clean up before more logic will be added to this
verification.

This clean up allows more single-purpose functions, and helper
functions to be located closer to the functions they help (as we
enforce an alphabetical ordering).

Bug: #51592
Change-Id: I7580deaaf00ca3731bf5de4e3ac53055aa81f99f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/291589
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>

Author: srawlins