Adopt faster traverseModules implementation in SwiftPM

Opening the SourceKit-LSP workspace using a debug build of sourcekit-lsp takes about 35s because `traverseModules` generates about 800,000 callback calls. All SourceKit-LSP needs now are the modules and their dependencies, so we can refactor `traverseModules` to just return that information avoid visiting the same modules multiple times. With theses changes getting all modules and their dependencies only takes ~0.013s.

rdar://136107035

Author: ahoppen

Sources/BuildSystemIntegration/SwiftPMBuildSystem.swift

@@ -205,9 +205,8 @@ package actor SwiftPMBuildSystem: BuiltInBuildSystem {
   private var buildDescription: SourceKitLSPAPI.BuildDescription?
 
   /// Maps target ids to their SwiftPM build target.
-  private var swiftPMTargets: [BuildTargetIdentifier: SwiftBuildTarget] = [:]
-
-  private var targetDependencies: [BuildTargetIdentifier: Set<BuildTargetIdentifier>] = [:]
+  private var swiftPMTargets:
+    [BuildTargetIdentifier: (target: SwiftBuildTarget, dependencies: Set<BuildTargetIdentifier>)] = [:]
 
   static package func projectRoot(
     for path: TSCBasic.AbsolutePath,
@@ -402,19 +401,16 @@ package actor SwiftPMBuildSystem: BuiltInBuildSystem {
     /// with only some properties modified.
 
     self.swiftPMTargets = [:]
-    self.targetDependencies = [:]
 
-    buildDescription.traverseModules { buildTarget, parent, depth in
+    buildDescription.traverseModules { buildTarget, dependencies in
       let targetIdentifier = orLog("Getting build target identifier") { try BuildTargetIdentifier(buildTarget) }
       guard let targetIdentifier else {
         return
       }
-      if let parent,
-        let parentIdentifier = orLog("Getting parent build target identifier", { try BuildTargetIdentifier(parent) })
-      {
-        self.targetDependencies[parentIdentifier, default: []].insert(targetIdentifier)
+      let dependencies = dependencies.compactMap { dependency in
+        orLog("Getting build target identifier of dependency") { try BuildTargetIdentifier(dependency) }
       }
-      swiftPMTargets[targetIdentifier] = buildTarget
+      swiftPMTargets[targetIdentifier] = (buildTarget, Set(dependencies))
     }
 
     connectionToSourceKitLSP.send(OnBuildTargetDidChangeNotification(changes: nil))
@@ -454,18 +450,18 @@ package actor SwiftPMBuildSystem: BuiltInBuildSystem {
   package func buildTargets(request: WorkspaceBuildTargetsRequest) async throws -> WorkspaceBuildTargetsResponse {
     var targets = self.swiftPMTargets.map { (targetId, target) in
       var tags: [BuildTargetTag] = [.test]
-      if !target.isPartOfRootPackage {
+      if !target.target.isPartOfRootPackage {
         tags.append(.dependency)
       }
       return BuildTarget(
         id: targetId,
-        displayName: target.name,
+        displayName: target.target.name,
         baseDirectory: nil,
         tags: tags,
         capabilities: BuildTargetCapabilities(),
         // Be conservative with the languages that might be used in the target. SourceKit-LSP doesn't use this property.
         languageIds: [.c, .cpp, .objective_c, .objective_cpp, .swift],
-        dependencies: self.targetDependencies[targetId, default: []].sorted { $0.uri.stringValue < $1.uri.stringValue },
+        dependencies: target.dependencies.sorted { $0.uri.stringValue < $1.uri.stringValue },
         dataKind: .sourceKit,
         data: SourceKitBuildTarget(toolchain: toolchain.path?.asURI).encodeToLSPAny()
       )
@@ -503,7 +499,7 @@ package actor SwiftPMBuildSystem: BuiltInBuildSystem {
           )
         )
       }
-      guard let swiftPMTarget = self.swiftPMTargets[target] else {
+      guard let swiftPMTarget = self.swiftPMTargets[target]?.target else {
         continue
       }
       var sources = swiftPMTarget.sources.map {
@@ -535,7 +531,7 @@ package actor SwiftPMBuildSystem: BuiltInBuildSystem {
       return try settings(forPackageManifest: path)
     }
 
-    guard let swiftPMTarget = self.swiftPMTargets[request.target] else {
+    guard let swiftPMTarget = self.swiftPMTargets[request.target]?.target else {
       logger.error("Did not find target \(request.target.forLogging)")
       return nil
     }
@@ -630,7 +626,7 @@ package actor SwiftPMBuildSystem: BuiltInBuildSystem {
     logMessageToIndexLog(
       taskID,
       """
-      Preparing \(self.swiftPMTargets[target]?.name ?? target.uri.stringValue)
+      Preparing \(self.swiftPMTargets[target]?.target.name ?? target.uri.stringValue)
       \(arguments.joined(separator: " "))
       """
     )