[7.x] Provide psr/container-implementation

[AegirLeet]

See laravel/ideas#2241.

As recommended in PSR-11, this declares that illuminate/container provides the psr/container-implementation virtual package.

[GrahamCampbell]

This must be added to the root composer.json file too.

[AegirLeet]

Doesn't it propagate because laravel/framework replaces illuminate/container? symfony/symfony doesn't provide it, but it's listed here, so I assumed that's how it works.