…or service in an injected service locator argument (XuruDragon, nicolas-grekas)

This PR was merged into the 4.3-dev branch.

Discussion
----------

[DependencyInjection] Add ability to define an index for service in an injected service locator argument

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | in progress /  symfony/symfony-docs#...

It's more or less the same thing then the PR #30257 but for a service locator argument

Add a simple way to specify an index based on a tag attribute to simplify retrieving a specific service when injecting a service locator as argument into services, but also a way to fallback to a static method on the service class.

Yaml:
```yaml
services:
  foo_service:
    class: Foo
    tags:
      - {name: foo_tag, key: foo_service}
  foo_service_tagged:
    class: Bar
    arguments:
      - !tagged_locator
          tag: 'foo_tag'
          index_by: 'key'
          default_index_method: 'static_method'
```
XML:
```xml
<?xml version="1.0" ?>

<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://symfony.com/schema/dic/services
    http://symfony.com/schema/dic/services/services-1.0.xsd">
  <services>
    <service id="foo" class="Foo">
        <tag name="foo_tag" key="foo_service" />
    </service>
    <service id="foo_tagged_iterator" class="Bar" public="true">
      <argument type="tagged_locator" tag="foo_tag" index-by="key" default-index-method="static_method" />
    </service>
  </services>
</container>
```
PHP:
```php
// config/services.php
use Symfony\Component\DependencyInjection\Argument\TaggedIteratorArgument;
use Symfony\Component\DependencyInjection\Argument\ServiceLocatorArgument;

$container->register(Foo::class)
        ->addTag('foo_tag', ['key' => 'foo_service']);

$container->register(App\Handler\HandlerCollection::class)
         // inject all services tagged with app.handler as first argument
         ->addArgument(new ServiceLocatorArgument(new TaggedIteratorArgument('app.handler', 'key')));
```

Usage:
```php
// src/Handler/HandlerCollection.php
namespace App\Handler;

use Symfony\Component\DependencyInjection\ServiceLocator;

class HandlerCollection
{
     public function __construct(ServiceLocator $serviceLocator)
     {
           $foo = $serviceLocator->get('foo_service'):
     }
}
```

Tasks

* [x]  Support PHP loader/dumper
* [x]  Support YAML loader/dumper
* [x]  Support XML loader/dumper (and update XSD too)
* [x]  Add tests
* [x]  Documentation

Commits
-------

cb3c56b Support indexing tagged locators by FQCN as fallback
250a2c8 [DI] Allow tagged_locator tag to be used as an argument

…or service locators (Anthony MARTIN)

This PR was submitted for the master branch but it was merged into the 4.3 branch instead (closes #11042).

Discussion
----------

[DependencyInjection] Doc for Allow to choose an index for service locators

Here is the doc for the new feature implemented in : [symfony/symfony#30348](symfony/symfony#30348) that follow the feature [symfony/symfony#30257](symfony/symfony#30257)

Commits
-------

d63c298 [DependencyInjection] Doc for Allow to choose an index for service locator collection