Handle filesDependenciesUpdated in BuildSystemManager instead of the build system

The implementation of which file’s dependencies have been updated is common across all build systems and thus build systems shouldn’t need to implement this logic. This also allows us to remove `BuildSystemDelegate`.

Sources/BuildSystemIntegration/FallbackBuildSystem.swift

@@ -38,13 +38,6 @@ package actor FallbackBuildSystem {
     )
   }()
 
-  /// Delegate to handle any build system events.
-  package weak var delegate: BuildSystemDelegate? = nil
-
-  package func setDelegate(_ delegate: BuildSystemDelegate?) async {
-    self.delegate = delegate
-  }
-
   package var indexStorePath: AbsolutePath? { return nil }
 
   package var indexDatabasePath: AbsolutePath? { return nil }