This PR was merged into the 4.3-dev branch.

Discussion
----------

Alias for each assets package

| Q             | A
| ------------- | ---
| Branch?       | master <!-- see below -->
| Bug fix?      | no
| New feature?  | yes <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | ?     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets |   <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | <!-- required for new features -->

Add autowiring by type + name on assets packages
```yaml
framework:
    assets:
        packages:
            xxx:
                base_urls: 'xxxx'
```
```php
<?php

class MyService
{
    private $xxxPackage;

    public function __construct(PackageInterface $xxxPackage)
    {
        $this->xxxPackage = $xxxPackage;
        ...
    }

    public function myMethod(): string
    {
        return $this->xxxPackage->getUrl('some-image.png');
    }
}
```
instead of:
```php
<?php

class MyService
{
    private $packages;

    public function __construct(Packages $packages)
    {
        $this->packages = $packages;
        ...
    }

    public function myMethod(): string
    {
        return $this->packages->getPackage('xxx')->getUrl('some-image.png');
    }
}
```

Commits
-------

e8b9856 Alias for each assets package