Expose SourceKitLSP library product

It's not at all stable, so underscore it, but it may be useful for
experimentation.

Author: benlangmuir

Package.swift

@@ -9,6 +9,11 @@ let package = Package(
         name: "sourcekit-lsp",
         targets: ["sourcekit-lsp"]
       ),
+      .library(
+        name: "_SourceKitLSP",
+        type: .dynamic,
+        targets: ["SourceKitLSP"]
+      ),
       .library(
         name: "LSPBindings",
         type: .static,