minor #36670 [Translations] Throw exception if xFileLoader dependencies don't exist. (jrushlow)

This PR was squashed before being merged into the 5.1-dev branch.

Discussion
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[Translations] Throw exception if xFileLoader dependencies don't exist.

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | Fix #36658
| License       | MIT
| Doc PR        |

`XliffFileLoader` & `QtFileLoader` both require `XmlUtils::class` from the `Config` component. This PR throws a friendly exception is the `Config` component does not exist.

Original idea by @xabbuh was to throw the exception from the __constructor. This PR throws the exception from the `load()` method to be consistent with the `YamlFileLoader::class`.  But that can easily be changed.

Commits
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627e476eb4 [Translations] Throw exception if xFileLoader dependencies don't exist.

src/Symfony/Component/Translation/Loader/QtFileLoader.php

@@ -15,6 +15,7 @@ use Symfony\Component\Config\Resource\FileResource;
 use Symfony\Component\Config\Util\XmlUtils;
 use Symfony\Component\Translation\Exception\InvalidResourceException;
 use Symfony\Component\Translation\Exception\NotFoundResourceException;
+use Symfony\Component\Translation\Exception\RuntimeException;
 use Symfony\Component\Translation\MessageCatalogue;
 
 /**
@@ -29,6 +30,10 @@ class QtFileLoader implements LoaderInterface
      */
     public function load($resource, string $locale, string $domain = 'messages')
     {
+        if (!class_exists(XmlUtils::class)) {
+            throw new RuntimeException('Loading translations from the QT format requires the Symfony Config component.');
+        }
+
         if (!stream_is_local($resource)) {
             throw new InvalidResourceException(sprintf('This is not a local file "%s".', $resource));
         }

src/Symfony/Component/Translation/Loader/XliffFileLoader.php

@@ -15,6 +15,7 @@ use Symfony\Component\Config\Resource\FileResource;
 use Symfony\Component\Config\Util\XmlUtils;
 use Symfony\Component\Translation\Exception\InvalidResourceException;
 use Symfony\Component\Translation\Exception\NotFoundResourceException;
+use Symfony\Component\Translation\Exception\RuntimeException;
 use Symfony\Component\Translation\MessageCatalogue;
 use Symfony\Component\Translation\Util\XliffUtils;
 
@@ -30,6 +31,10 @@ class XliffFileLoader implements LoaderInterface
      */
     public function load($resource, string $locale, string $domain = 'messages')
     {
+        if (!class_exists(XmlUtils::class)) {
+            throw new RuntimeException('Loading translations from the Xliff format requires the Symfony Config component.');
+        }
+
         if (!stream_is_local($resource)) {
             throw new InvalidResourceException(sprintf('This is not a local file "%s".', $resource));
         }