…ackages (GromNaN)

This PR was merged into the 5.3-dev branch.

Discussion
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[Framework] Add tag assets.package to register asset packages

Replaces #38366

| Q             | A
| ------------- | ---
| Branch?       | 5.x
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | symfony/symfony-docs#14962

To configure asset packages in an application, we have to declare it in the `framework` configuration ([doc for assets config](https://symfony.com/doc/current/reference/configuration/framework.html#assets)). In some case we cannot use this configuration:
- To use a custom class as package
- To register an asset package in a shared bundle (my use-case).

This PR adds the `assets.package` tag. This tag is use to collect and inject package services into the `assets.packages` service, that is the registry for all packages. Since every package needs a name, the `package` attribute of the tag is used (same naming convention that the `console.command` tag).

Main changes:
- the packages defined in the `framework.assets` configuration are injected into the `assets.packages` using the tag instead of being directly injected in service declaration.
- changed signature of `Symfony\Components\Assets\Packages` constructor to accept an iterator (backward compatible).
- a new alias `assets._default_package` is defined even if assets are not configured.

### Example in `symfony/demo` ([commit](symfony/demo@e5e5a8f...GromNaN:assets-package-tag)):

In `config/services.yaml`:
```yaml
    avatar.strategy:
        class: Symfony\Component\Asset\VersionStrategy\JsonManifestVersionStrategy
        arguments:
            - '%kernel.project_dir%/public/build/manifest.json'

    avatar.package:
        class:  Symfony\Component\Asset\Package
        arguments:
            - '@avatar.strategy'
            - '@assets.context'
        tags:
            - { name: assets.package, package: avatars }
```

Then we can use the package anywhere
```twig
<img src="{{ asset('anna.jpg', 'avatars') }}">
```

### Alternative using autoconfiguration with a custom class:

With a custom class implementing the `PackageInterface`, the package name can be provided by a the static method `getDefaultPackageName`. Autowiring and autoconfiguration will import the package.

```php
namespace App\Asset;

use Symfony\Component\Asset\PackageInterface;

class AvatarPackage implements PackageInterface
{
    public static function getDefaultPackageName(): string
    {
        return 'avatars';
    }

    // ... Implements the interface
}
```

Commits
-------

6217ff7 [Asset] Add tag assets.package to register asset packages