If a source file is part of multiple targets, we should index it in the context of all of those targets because the different targets may produce different USRs since they might use different build settings to interpret the file.
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.
