Plugins should be registered as build extensions

[cstamas]

For this to work, plugins must be registered as build extensions <extensions>true</extensions> see https://gist.github.com/cstamas/191a6bb660c7b1cca1a325248af30d1f

Once apache/maven-install-plugin#15 and apache/maven-deploy-plugin#20 is merged/released, then this trick above will not be needed anymore.

Reason: current "naive" atEnd implementation uses static member https://github.com/apache/maven-install-plugin/blob/master/src/main/java/org/apache/maven/plugins/install/InstallMojo.java#L58 but as Mojo class-loaders are re-created per module (by different conditions), the static is "lost" as in case of new class-loader, the old data is not there. Hence, as extension force Maven to load plugins once, and keep it throughout the build, and that makes static member "live" thru whole build,