Part of https://github.com/elastic/elasticsearch/issues/106679
* Copy the `ql` project into a different project _just for esql_, call it `esql-core`.
* Make `esql` depend only on the latter.
* Fix `EsqlNodeSubclassTests`; I'm confused why this didn't bite us earlier.
* Update the warning regexes in some csv tests as the exceptions have other package names now.
**Note to reviewers:** Exclude the first commit when viewing the diff,
as that contains only the actual copying of `ql`. The remaining commits
are the actually meaningful ones. _The `build.gradle` files probably
require the most attention._
