Fix cyclic dependency between xpack-core and license-tools modules

[martijnvg]

Currently xpack-core module depends on the license-tools module and the license-tools module depends on xpack-core module.

I think to remove the cyclic dependency, the org.elasticsearch.license.licensor.LicenseSigner class should be moved from license-tools to xpack-core; then the xpack-core will no longer need to depend on license-tools?

Originates from #41064.

[elasticmachine]

Pinging @elastic/es-core-infra

[alpar-t]

This is a cyclic dependency only in eclipse terms that does not distinguish between main and test dependencies. x-pack tests depend on license-tools.
This will no longer be relevant if #44075 merges.

[mark-vieira]

@nik9000 is this still a concern or is Eclipse with Buildship able to cope with this now?

[nik9000]

Its now part of our standard Eclipse configuration to switch loops in the dependencies from build failures to build warnings. Its a manual step you have to do after importing the project. So, yeah, we can ignore this. Its pretty low down the list of things weird about eclipse.