If you're using the generated project, you need to use our xcconfig overrides to add the link to CoreServices:
swift package generate-xcodeproj --xcconfig-overrides overrides.xcconfig
That being said, if you're using the Xcode 11 beta, you could also build sourcekit-lsp directly by using the swift package support that was added to Xcode by opening the Package.swift file rather than generating an xcodeproj.