LOG4J2-741 Reinstate the package configuration attribute for discovering custom plugins

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1613615 13f79535-47bb-0310-9956-ffa450edef68
small punctuation change

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1613621 13f79535-47bb-0310-9956-ffa450edef68