This finalizes the move of `BuiltInBuildSystem` creation into `BuiltInBuildSystemAdapter` and means that we can set the message handler of the `BuiltInBuildSystem` during initialization instead of using a setter method.
This way we create the `BuiltInBuildSystem` at the same time that we create the `BuildSystemManager`, which gets us one step closer to creating the `BuiltInBuildSystem` from the `BuiltInBuildSystemAdapter`.