Conflict with lespoilus/spyc when installed with composer

When I install wp-cli with composer, and I have already installed "lespoilus/spyc" librairy in composer, the following error is triggered when I try to execute the "vendor/bin/wp" command : 
```sh
$ ./vendor/bin/wp

PHP Fatal error:  Cannot redeclare class Spyc in vendor/wp-cli/wp-cli/php/Spyc.php on line 57 
```

The vendor/composer/autoload_files.php  file contains twice "spyc.php"

```php
return array(
    $vendorDir . '/lespoilus/spyc/spyc.php',
    $vendorDir . '/wp-cli/php-cli-tools/lib/cli/cli.php',
    $vendorDir . '/wp-cli/wp-cli/php/Spyc.php',
);
```

Would not it be better to add a dependency on "lespoilus/spyc"  in your composer.json ?

Thanks.