_markImportersAsChanged(filePath) {
      if (importedByMap.has(filePath)) {
        const importedBy = importedByMap.get(filePath);
        importedByMap.delete(filePath);
        for (const importerFilePath of importedBy) {
          this.markChanged(importerFilePath);
        }
      }
    }
If any files imported the given file path, mark them as changed.
@private