This PR was merged into the 3.4 branch.

Discussion
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[DI] Reference tagged services in config

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12269
| License       | MIT
| Doc PR        | symfony/symfony-docs#8404

This is a proof of concept to reference a sequence of tagged services.

The problem bugs me for some time, and at first i thought the solution was to have some super generic compiler pass. If it could replace a lot of compilers in core.. perhaps worth it, but eventually each tag comes with it's own logic, including how to deal with tag attributes.

However, writing the passes over and over again becomes tedious for the most basic usecase. So given the recent developments, this idea came to mind.

```yml
services:
    a:
        class: stdClass
        properties: { a: true }
        tags: [foo]

    b:
        class: stdClass
        properties: { b: true }
        tags: [foo]

    c:
        class: stdClass
        properties:
            #stds: !tagged_services foo (see #22198)
            stds: !tagged_services
                foo
```

```
dump(iterator_to_array($this->get('c')->stds));
```

```
array:2 [▼
  0 => {#5052 ▼
    +"a": true
  }
  1 => {#4667 ▼
    +"b": true
  }
]
```

Given the _basic_ example at https://symfony.com/doc/current/service_container/tags.html, this could replace that.

Any thoughts?

Commits
-------

979e58f [DI] Reference tagged services in config