I don't want to dig too closely into the ES integration yet because
it being fixed by Costin so instead I'll make an example for round
trip testing.
Original commit: elastic/x-pack-elasticsearch@4d8fbc571e
Make the projects more like the rest of Elasticsearch:
* Move integration tests from `itest` to `test`
* Make a build file per sql subproject
* Make sql work properly with elasticsearch-extra
Original commit: elastic/x-pack-elasticsearch@6fbb794f68