Build: Move xpack to plugin group

This is needed in order to make `bin/elasticsearch-plugin install xpack`
work, as it expects the plugin in a certain path.

Original commit: elastic/x-pack-elasticsearch@252c55e5a8

elasticsearch/x-pack/build.gradle

@@ -1,6 +1,8 @@
 import org.elasticsearch.gradle.MavenFilteringHack
 import org.elasticsearch.gradle.test.NodeInfo
 
+group 'org.elasticsearch.plugin'
+
 apply plugin: 'elasticsearch.esplugin'
 esplugin {
   name 'xpack'