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/BuildServerBuildSystem.swift

@@ -263,7 +263,11 @@ extension BuildServerBuildSystem: BuildSystem {
   ///
   /// Returns `nil` if no build settings have been received from the build
   /// server yet or if no build settings are available for this file.
-  public func buildSettings(for document: DocumentURI, language: Language) async -> FileBuildSettings? {
+  public func buildSettings(
+    for document: DocumentURI,
+    in target: ConfiguredTarget,
+    language: Language
+  ) async -> FileBuildSettings? {
     return buildSettings[document]
   }
 
@@ -271,6 +275,10 @@ extension BuildServerBuildSystem: BuildSystem {
     return nil
   }
 
+  public func configuredTargets(for document: DocumentURI) async -> [ConfiguredTarget] {
+    return [ConfiguredTarget(targetID: "dummy", runDestinationID: "dummy")]
+  }
+
   public func registerForChangeNotifications(for uri: DocumentURI) {
     let request = RegisterForChanges(uri: uri, action: .register)
     _ = self.buildServer?.send(request) { result in