Make bits of ModuleXmlParser less non-public

WildFly Swarm has had to create a ModuleXmlParserBridge in order to gain access to some bits of ModuleXmlParser we've needed.

https://github.com/wildfly-swarm/wildfly-swarm/blob/master/bootstrap/src/main/java/org/jboss/modules/ModuleXmlParserBridge.java

Would be lovely if we didn't have to do this.