if (!this._graph.dependencies.size) {
      const {added} = await this._graph.initialTraverseDependencies(
        this._options,
      );

      return {
        added,
        modified: new Map(),
        deleted: new Set(),
        reset: true,
      };
    }
Handles a single file change. To avoid doing any work before it's needed,
the listener only stores the modified file, which will then be used later
when the delta needs to be calculated.