Support use of hyphen in asset package name

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no
| Tests pass?   | yes (Manual tests only)
| Fixed tickets | #28122
| License       | MIT
| Doc PR        | n/a

According to issue symfony/symfony-docs#10442, we tested in a demo bundle, for example in src/AppBundle/Resources/config/config.yml a package using hyphens: app-client-frontend, and withouth the patch it fails because the package is not recognized. With the patch, it works as expected.
```
framework:
    assets:
        packages:
            app-client-frontend:
                version: "%env(FRONTEND_VERSION)%"
                version_format: '%%2$s/dist/%%1$s'
                base_urls:
                  - "%env(FRONTEND_URL)%"
```

Author: damaya

DependencyInjection/Configuration.php

@@ -642,6 +642,7 @@ private function addAssetsSection(ArrayNodeDefinition $rootNode)
                     ->fixXmlConfig('package')
                     ->children()
                         ->arrayNode('packages')
+                            ->normalizeKeys(false)
                             ->useAttributeAsKey('name')
                             ->prototype('array')
                                 ->fixXmlConfig('base_url')