Rename package org.apache.logging.log4j.core.helpers to org.apache.logging.log4j.core.util.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1594988 13f79535-47bb-0310-9956-ffa450edef68
Use constant.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1595003 13f79535-47bb-0310-9956-ffa450edef68