Introduce a notion of ConfiguredTargets into the build system

Instead of asking for build settings of a file, the build system manager asks for the targets of a file and then asks for the build settings of that file in a specific target. This has two advantages:
- We know about targets and can prepare the targets for background indexing
- Once we support build systems in which a single file can be part of multiple targets, we can have a centralized place that picks preferred targets for a file, eg. based on user configuration

Sources/SKCore/CompilationDatabaseBuildSystem.swift

@@ -93,14 +93,17 @@ public actor CompilationDatabaseBuildSystem {
 }
 
 extension CompilationDatabaseBuildSystem: BuildSystem {
-
   public var indexDatabasePath: AbsolutePath? {
     indexStorePath?.parentDirectory.appending(component: "IndexDatabase")
   }
 
   public var indexPrefixMappings: [PathPrefixMapping] { return [] }
 
-  public func buildSettings(for document: DocumentURI, language: Language) async -> FileBuildSettings? {
+  public func buildSettings(
+    for document: DocumentURI,
+    in buildTarget: ConfiguredTarget,
+    language: Language
+  ) async -> FileBuildSettings? {
     guard let url = document.fileURL else {
       // We can't determine build settings for non-file URIs.
       return nil
@@ -118,6 +121,10 @@ extension CompilationDatabaseBuildSystem: BuildSystem {
     return nil
   }
 
+  public func configuredTargets(for document: DocumentURI) async -> [ConfiguredTarget] {
+    return [ConfiguredTarget(targetID: "dummy", runDestinationID: "dummy")]
+  }
+
   public func registerForChangeNotifications(for uri: DocumentURI) async {
     self.watchedFiles.insert(uri)
   }