We must edit maven-navigation.xsd in the ${plugin.resources}/xsd directory
but never maven-navigation-${currentRelease}.xsd in src/xsd.

Every time a new version of the plugin is released, we copy
${plugin.resources}/xsd/maven-navigation.xsd into
src/xsd/maven-navigation-${currentRelease}.xsd.

We store maven-navigation-${currentRelease}.xsd in the SCM to be able
to re-publish them if necessary.