This is the next step in removing the top level sql directory.
I named the directory `sql-cli` instead of `cli` because that puts it at
the maven coordinates `org.elasticsearch.plugin:sql-cli` instead of
`org.elasticsearch.plugin:cli`.
Relates to elastic/x-pack-elasticsearch#3363
Original commit: elastic/x-pack-elasticsearch@d41a57a136
Nik Everett