I18N: Support loading .l10n.php translation files on their own.

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

tests/phpunit/data/plugins/internationalized-plugin-2.php

@@ -0,0 +1,12 @@
+<?php
+/*
+Plugin Name: Dummy Plugin 2
+Plugin URI: https://wordpress.org/
+Description: For testing purposes only. Only has an .l10n.php translation file.
+Version: 1.0.0
+Text Domain: internationalized-plugin
+*/
+
+function i18n_plugin_2_test() {
+	return __( 'This is a dummy plugin', 'internationalized-plugin-2' );
+}