- Use conventional plugin naming, i.e. "detect-split-packages" instead
   of applying plugin based on fully-qualified class name

 - Rename "diagnose" => "detect" consistently throughout plugin, task
   and method names and generally refactor naming throughout to follow
   "detect split packages" phrasing

 - Add Javadoc to DetectSplitPackagesPlugin

 - Improve error reporting when split packages are detected

   Upon detecting one or more split packages, `detectSplitPackages` now
   fails idiomatically, throwing a GradleException to signal task failure
   (as opposed to the previous approach of using an assert assertion), and
   the output reads as follows:

       $ gradle detectSplitPackages
       [...]
       :buildSrc:build UP-TO-DATE
       :detectSplitPackages FAILED

       FAILURE: Build failed with an exception.

       * What went wrong:
       Execution failed for task ':detectSplitPackages'.
       > The following split package(s) have been detected:
          - org.springframework.beans (split across spring-beans and spring-orm)
          - org.springframework.core.env (split across spring-context and spring-core)

 - DetectSplitPackagesTask now automatically attaches itself to `check`
   task lifecycle if the enclosing project contains a `check` task

 - DetectSplitPackagesTask adds itself to the 'Verification' task group,
   ensuring that it shows up correctly in `gradle tasks` task listings

 - packagesToScan now defaults to all subprojects; users may then
   customize this by removing individual subprojects from the collection

Issue: SPR-9990