This allows 'latest' gradle version

The dependency section is not generated when no packages are given. This makes the config file easier to read for plugin-only configs.

* WIP Try to add variant support

* Rewrite package resolver

The new version adds support for multiple kotlin versions, and direct support for plugins. The total runtime is lower because the resolver does less work.

* Update README

Some dependencies are not catched by the quarkus bom, so it's
necessary to generate and build a project to cache them.

The new automatic test disables all external networking and tries to
generate and build the quarkus project.