add an option for MigrateListFiles to always append chunk IDs when working with multiple items per file

I am doing a migration where I have source files which contain body text, and I am extracting image files from IMG tags. I'm therefore using MigrateSourceList with MigrateListFiles as the list handler, to which I'm passing a custom subclass of MigrateContentParser. This analyses the body text, and extracts source IDs for the image files, which my migration can then use to retrieve the files from the site I'm importing from.

However, MigrateListFiles::getIDsFromFiles() only returns a chunk ID when a single file has multiple chunks. When a file happens to have only one IMG in the body, it returns just the file ID:

        if ($this->parser->getChunkCount() > 1) {
          foreach ($this->parser->getChunkIDs() as $chunk_id) {
            $ids[] = str_replace($this->baseDir, '', (string) $file->uri) . MIGRATE_CHUNK_SEPARATOR . $chunk_id;
          }
        }
        else {
          $ids[] = str_replace($this->baseDir, '', (string) $file->uri);
        }

This is a problem because it means in that case my MigrateItem handler (or my migration) needs to parse the original file all over again. This is not very clean, as it creates a special case.

It would be handy to have an option somewhere to cause all IDs to have the chunk ID appended.