This is the first step of refactoring Embark's pipeline abstraction into
dedicated plugin modules that take advantage of Embark's event system.
With this commit we're moving `Pipeline` into `lib/modules/pipeline` and
introduce a new command handler `pipeline:build`. Embark's engine now
requests builds via this command handler.
Notice that `Watch` still lives in `lib/pipeline` as this is a step-by-step
refactoring to reduce chances of introducing regressions.
Pascal Precht