Move non-annotations from plugins package to plugins.util package.

  - This prevents a package split due to log4j-plugin-processor.
  - In preparation for moving these annotations to the 
  log4j-plugin-processor module.
  - All references to these classes have been updated to match the 
  correct packages (and added imports where necessary).

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1588804 13f79535-47bb-0310-9956-ffa450edef68