Issue Description:

Right now, if SwiftPM doesn't find a system target's module.modulemap at the expected location, it emits the following diagnostic:

Source files for target SystemLib should be located under 'Sources/SystemLib', or a custom sources path can be set with the 'path' property in Package.swift

even if there is a module map file in that folder but with the wrong name. Perhaps SwiftPM should generate a more precise diagnostic:

missing system target module map at 'Sources/SystemLib/module.modulemap'

Or perhaps SwiftPM could also scan for any modulemap file inside the target folder.