[WIP] composer "extra config package" based bundle registration

[docteurklein]

Every library package that has configured the specified key in its extra config will be registered automatically:

{
    "name": "vendor/foo-bundle",
    "extra": {
        "register-bundles": ["Vendor\Bundle\VendorFooBundle"]
    }
}

The effect is to create a file at path app/bundle/vendor/foo-bundle that contains php code to instanciate the bundle:

<?php return new \Vendor\Bundle\VendorFooBundle($this);

The bundle always receive the kernel as first and only constructor argument.

A Kernel impl. may then be able to auto-discover and require these files to append them to its list of registered bundles.

[docteurklein]

rel symfony/symfony#9341

[ghost]

ongoing discussion about improving symfonly installation on symfony-standard. one of the approaches mentioned is close to the one proposed in this PR.

https://github.com/symfony/symfony-standard/issues/639