Build: Move xpack to plugin group
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@252c55e5a873518af3b8fc51604c5482710f6772

nknize/elasticsearch,vroyer/elassandra,gfyoung/elasticsearch,scorpionvicky/elasticsearch,robin13/elasticsearch,GlenRSmith/elasticsearch,strapdata/elassandra,gingerwizard/elasticsearch,gfyoung/elasticsearch,gingerwizard/elasticsearch,HonzaKral/elasticsearch,uschindler/elasticsearch,uschindler/elasticsearch,strapdata/elassandra,nknize/elasticsearch,coding0011/elasticsearch,vroyer/elassandra,GlenRSmith/elasticsearch,gingerwizard/elasticsearch,robin13/elasticsearch,scorpionvicky/elasticsearch,robin13/elasticsearch,scorpionvicky/elasticsearch,gfyoung/elasticsearch,strapdata/elassandra,GlenRSmith/elasticsearch,coding0011/elasticsearch,nknize/elasticsearch,coding0011/elasticsearch,vroyer/elassandra,coding0011/elasticsearch,gfyoung/elasticsearch,uschindler/elasticsearch,HonzaKral/elasticsearch,gingerwizard/elasticsearch,gfyoung/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,uschindler/elasticsearch,scorpionvicky/elasticsearch,uschindler/elasticsearch,HonzaKral/elasticsearch,GlenRSmith/elasticsearch,gingerwizard/elasticsearch,strapdata/elassandra,nknize/elasticsearch,robin13/elasticsearch,GlenRSmith/elasticsearch,scorpionvicky/elasticsearch,coding0011/elasticsearch,strapdata/elassandra,nknize/elasticsearch,HonzaKral/elasticsearch,gingerwizard/elasticsearch