Some pains have been taken within SourceKit to make `libdispatch` an optional dependency (e.g. the `WorkQueue` abstraction in `SourceKitSupport`, however it is neither used everywhere, nor are any implementations available besides the one that uses `libdispatch`. Since `libdispatch` is intended to be available anywhere Swift is anyway, it seems reasonable to go ahead and use it here, at least for now.
(Note that an alternative to this could be to remove `libdispatch` usage instead. See https://github.com/apple/swift/pull/2793)