* Build: Convert xplugins to use new extra projects setup
This change makes the gradle initialization for xplugins look in the
correct location for elasticsearch, which is now as a sibling of an
elasticsearch-extra directory, with x-plugins as a child of the extra
directory.
The elasticsearch side of this change is
elastic/elasticsearchelastic/elasticsearch#21773. This change will enable renaming x-plugins
to x-pack, see elastic/elasticsearch#3643.
Original commit: elastic/x-pack-elasticsearch@09398aea5a
This is a followup from elastic/elasticsearchelastic/elasticsearch#21590 and needs to be
committed first or at the same time since netty_3 is removed
Original commit: elastic/x-pack-elasticsearch@131d74dd6b