Improve performance of sourceFilesAndDirectories

`SourceFilesAndDirectoriesKey` contained all source files in the project and computing its hash value was pretty expensive. The key didn’t really provide any value here because the only way it changes is if the build targets change and if that’s the case, we already clear `cachedSourceFilesAndDirectories`, so we can just avoid the hash value computation.

Author: ahoppen

Sources/BuildSystemIntegration/BuildSystemManager.swift

@@ -327,11 +327,9 @@ package actor BuildSystemManager: QueueBasedMessageHandler {
 
   private var cachedTargetSources = RequestCache<BuildTargetSourcesRequest>()
 
-  /// The parameters with which `SourceFilesAndDirectories` can be cached in `cachedSourceFilesAndDirectories`.
-  private struct SourceFilesAndDirectoriesKey: Hashable {
-    let includeNonBuildableFiles: Bool
-    let sourcesItems: [SourcesItem]
-  }
+  /// `SourceFilesAndDirectories` is a global property that only gets reset when the build targets change and thus
+  /// has no real key.
+  private struct SourceFilesAndDirectoriesKey: Hashable {}
 
   private struct SourceFilesAndDirectories {
     /// The source files in the workspace, ie. all `SourceItem`s that have `kind == .file`.
@@ -1045,27 +1043,22 @@ package actor BuildSystemManager: QueueBasedMessageHandler {
   ///
   /// `includeNonBuildableFiles` determines whether non-buildable files should be included.
   private func sourceFilesAndDirectories(includeNonBuildableFiles: Bool) async throws -> SourceFilesAndDirectories {
-    let targets = try await self.buildTargets()
-    let sourcesItems = try await self.sourceFiles(in: Set(targets.keys))
-
-    let key = SourceFilesAndDirectoriesKey(
-      includeNonBuildableFiles: includeNonBuildableFiles,
-      sourcesItems: sourcesItems
-    )
+    return try await cachedSourceFilesAndDirectories.get(SourceFilesAndDirectoriesKey(), isolation: self) { key in
+      let targets = try await self.buildTargets()
+      let sourcesItems = try await self.sourceFiles(in: Set(targets.keys))
 
-    return try await cachedSourceFilesAndDirectories.get(key, isolation: self) { key in
       var files: [DocumentURI: SourceFileInfo] = [:]
       var directories: [DocumentURI: SourceFileInfo] = [:]
-      for sourcesItem in key.sourcesItems {
+      for sourcesItem in sourcesItems {
         let target = targets[sourcesItem.target]?.target
         let isPartOfRootProject = !(target?.tags.contains(.dependency) ?? false)
         let mayContainTests = target?.tags.contains(.test) ?? true
-        if !key.includeNonBuildableFiles && (target?.tags.contains(.notBuildable) ?? false) {
+        if !includeNonBuildableFiles && (target?.tags.contains(.notBuildable) ?? false) {
           continue
         }
 
         for sourceItem in sourcesItem.sources {
-          if !key.includeNonBuildableFiles && sourceItem.sourceKitData?.isHeader ?? false {
+          if !includeNonBuildableFiles && sourceItem.sourceKitData?.isHeader ?? false {
             continue
           }
           let info = SourceFileInfo(