New wrapper modules

The introduction of these new modules will very likely break the biggest part of existing projects using Drools engine. The trivial fix for these situations will be adding to the project's classpath drools-mvel and/or drools-serialization-protobuf according to the project's requirements.

This issue is mainly caused by the fact that users have among their dependencies modules like drools-core and drools-compiler that in reality are no more than "internal implementation details". To avoid similar problems in future, in the very likely case that other modules will be introduced to split engine in finer grained submodules and descope optional features, the following 2 new wrapper modules have been created to cover the 2 different main usage scenarios:

- drools-engine : aggregating drools-core, drools-compiler and drools-model-compiler

- drools-engine-classic : aggregating drools-core, drools-compiler and drools-mvel

With the introduction of these 2 modules, to use Drools engine with the executable model it will be enough to add drools-engine to a project's dependencies while for traditional use of Drools engine without the model it will be necessary to just import drools-engine-classic. These names reflect the intention of the Drools team to promote the use of the executable model and make it the default choice.