There is only one real class that implements the `SourceKitD` protocol, so there really isn’t any need for the protocol + class split at all. Unify them to make code simpler to reason about.
This adds a sourcekitd plugin that drives the code completion requests. It also includes a `CompletionScoring` module that’s used to rank code completion results based on their contextual match, allowing us to show more relevant code completion results at the top.
