refactoring: move CheckstyleExecutor coponent to its a new dedicated exec package

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1607987 13f79535-47bb-0310-9956-ffa450edef68
refactoring: move CheckstyleExecutor coponent to its a new dedicated exec package

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1607988 13f79535-47bb-0310-9956-ffa450edef68