Remove dependency from SourceKitLSP on swift-docc-symbolkit

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

Package.swift

@@ -509,7 +509,6 @@ var targets: [Target] = [
       .product(name: "Crypto", package: "swift-crypto"),
       .product(name: "Markdown", package: "swift-markdown"),
       .product(name: "SwiftToolsSupport-auto", package: "swift-tools-support-core"),
-      .product(name: "SymbolKit", package: "swift-docc-symbolkit"),
     ]
       + swiftPMDependency([
         .product(name: "SwiftPM-auto", package: "swift-package-manager")

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)
+}

Sources/SourceKitLSP/Documentation/DoccDocumentationHandler.swift

@@ -19,7 +19,6 @@ package import LanguageServerProtocol
 import Markdown
 import SKUtilities
 import SemanticIndex
-import SymbolKit
 
 extension DocumentationLanguageService {
   package func doccDocumentation(_ req: DoccDocumentationRequest) async throws -> DoccDocumentationResponse {
@@ -107,23 +106,9 @@ extension DocumentationLanguageService {
       // Create a dummy symbol graph and tell SwiftDocC to convert the module name.
       // The version information isn't really all that important since we're creating
       // what is essentially an empty symbol graph.
-      let emptySymbolGraph = String(
-        data: try JSONEncoder().encode(
-          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: []
-          )
-        ),
-        encoding: .utf8
-      )
       return try await documentationManager.renderDocCDocumentation(
         symbolUSR: moduleName,
-        symbolGraph: emptySymbolGraph,
+        symbolGraph: emptySymbolGraph(forModule: moduleName),
         markupFile: snapshot.text,
         moduleName: moduleName,
         catalogURL: catalogURL