This change moves the Java code from `/java/apps/engine-node` to `/elasticsearch` so it matches the structure required by X-Pack. This will make it easier to get the gradle build working correct and building the full Pack zip.
Original commit: elastic/x-pack-elasticsearch@2fd6539e85
They'll soon cease to work at all when $PRELERT_HOME is removed
Any remaining code that needs to be ported can be copied from the analytics-dev branch
Original commit: elastic/x-pack-elasticsearch@1f2d97b429