We need a mapping from source file to its output path in order to support source files that are part of multiple targets (because we need the output path to check if we have an up-to-date unit for a file in a given target). To achieve this mapping, it’s easier to tag the output path for each source file onto the `buildTarget/sources` request.
I feel like the implementations are actually simpler if we split them. This will also allow us to add more advanced logic to the JSON compilation database build system in the future, such as inferring the toolchain from the compile command.
