`buildTarget/inverseSources` is not required to be implemented by BSP servers and we have almost all information needed for it in `BuildSystemManager`.
This also makes sure that `buildTarget/sources` and `buildTarget/inverseSources` actually match each other. Before this change, we had source files like `Package.swift` for which we returned a target from `buildTarget/inverseSources` but that weren’t part of that target’s sources retrieved using `buildTarget/sources`.
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.
