Remove dependency from SourceKitLSP on swift-docc-symbolkit

This way all dependencies on the `docc` libraries are wrapped inside the `DocCDocumentation` module.

Sources/DocCDocumentation/EmptySymbolGraph.swift

@@ -0,0 +1,17 @@
+import Foundation
+import SymbolKit
+
+/// Generates a JSON string that represents an empty symbol graph for the given module name.
+package func emptySymbolGraph(forModule moduleName: String) throws -> String? {
+  let symbolGraph = SymbolGraph(
+    metadata: SymbolGraph.Metadata(
+      formatVersion: SymbolGraph.SemanticVersion(major: 0, minor: 0, patch: 0),
+      generator: "SourceKit-LSP"
+    ),
+    module: SymbolGraph.Module(name: moduleName, platform: SymbolGraph.Platform()),
+    symbols: [],
+    relationships: []
+  )
+  let data = try JSONEncoder().encode(symbolGraph)
+  return String(data: data, encoding: .utf8)
+}