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.
I feel like the implementations are actually simpler if we split them. This will also allow us to add more advanced logic to the JSON compilation database build system in the future, such as inferring the toolchain from the compile command.
We need to jump through a few extra hoops when the SourceKit plugin is located in SourceKit-LSP’s build folder and we are using `sourcekitd_plugin_initialize` instead of `sourcekitd_plugin_initialize_2`.
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.
