Modularize score directors

Score directors are tightly coupled to core. That prevents users from excluding any which they do not need. As a result, users of incremental score calculator need to drag all of Drools with them.

The goal of this epic is to create an SPI for score directors, so that each score director can sit in its own optional JAR.

ScoreDirectors which we will eventually want to make optional:

• CS (Both Drools and Bavet.)
• DRL

Incremental and Easy will be kept in core, since they require no additional dependencies.