Перед App попробуйте \ поставить
$this->app->bind('\App\Contracts\DataSource', function ($app) {
return new \App\DataSources\Source;
});
или
use App\Contracts\DataSource;
use App\DataSources\Source;
и
$this->app->bind(DataSource::class, function ($app) {
return new Source::class;
});