Adjusts the translation file lookup in `WP_Textdomain_Registry` so that just-in-time translation loading
works even if there is only a `.l10n.php` translation file without a corresponding `.mo` file.
While language packs continue to contain both file types, this makes it easier to use translations in a project
without having to deal with `.mo` or `.po` files.
Props Chrystl.
See #59656.
git-svn-id: https://develop.svn.wordpress.org/trunk@57516 602fd350-edb4-49c9-b593-d223f7449a82
