bug #14325 [Routing][DependencyInjection] Support .yaml extension in YAML loaders (thunderer)

This PR was squashed before being merged into the 2.3 branch (closes #14325).

Discussion
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[Routing][DependencyInjection] Support .yaml extension in YAML loaders

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #14319
| License       | MIT
| Doc PR        | -

YAML [FAQ](http://www.yaml.org/faq.html) states that .yaml file extension should be used whenever possible. I tweaked YamlFileLoader supports() method in Symfony Routing component to accept both .yml and .yaml and added some asserts in tests for that behavior.

This PR replaces #14319 as it was based on 2.7. BTW Is there a way to "rebase" PR branch without redoing all the work?

Commits
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dd5a811 [Routing][DependencyInjection] Support .yaml extension in YAML loaders

Author: Tobion

Loader/YamlFileLoader.php

@@ -74,7 +74,7 @@ public function load($resource, $type = null)
      */
     public function supports($resource, $type = null)
     {
-        return is_string($resource) && 'yml' === pathinfo($resource, PATHINFO_EXTENSION);
+        return is_string($resource) && in_array(pathinfo($resource, PATHINFO_EXTENSION), array('yml', 'yaml'), true);
     }
 
     /**

Tests/Loader/YamlFileLoaderTest.php

@@ -187,6 +187,7 @@ public function testSupports()
         $loader = new YamlFileLoader(new ContainerBuilder(), new FileLocator());
 
         $this->assertTrue($loader->supports('foo.yml'), '->supports() returns true if the resource is loadable');
+        $this->assertTrue($loader->supports('foo.yaml'), '->supports() returns true if the resource is loadable');
         $this->assertFalse($loader->supports('foo.foo'), '->supports() returns true if the resource is loadable');
     }