Compliance with PSR-4 autoloading

[Epse]

With Composer 2 being released, this project will need to comply with PSR-4.
It's not entirely clear to me what part does not comply, but composer complains about src/Contracts/Searchable/Searchable.php.

[marnickmenting]

I am running composer 2.0.8 and get no errors.

[s00d]

problems connected with composer version:

github:

namespace Sofa\Eloquence\Contracts\Searchable;

composer:

namespace Sofa\Eloquence\Searchable;

Please create new version