changed plugin to extension, since this is the only useful part

git-svn-id: https://svn.apache.org/repos/asf/maven/shared/trunk@1597191 13f79535-47bb-0310-9956-ffa450edef68
fixed typo

git-svn-id: https://svn.apache.org/repos/asf/maven/shared/trunk@1597201 13f79535-47bb-0310-9956-ffa450edef68
hardened checks

git-svn-id: https://svn.apache.org/repos/asf/maven/shared/trunk@1597206 13f79535-47bb-0310-9956-ffa450edef68