SQL: Move all SQL projects into org.elasticsearch.sql package (elastic/x-pack-elasticsearch#3069)

Elasticsearch plugins are traditionally using org.elasticsearch.plugin group, clients are using org.elasticsearch.client, so it makes sense to move all sql pieces and deliverables into org.elasticsearch.sql group. Otherwise, users might get really confused about an artifact named `server` in the group `org.elasticsearch`.

Original commit: elastic/x-pack-elasticsearch@c3a4042040

sql/build.gradle

@@ -1,6 +1,8 @@
 description = 'SQL for Elasticsearch'
 
 subprojects {
+  group = 'org.elasticsearch.sql'
+
   apply plugin: 'elasticsearch.build'
 
   dependencies {