Class Wagnerwagner\Merx\Cart located in ./site/plugins/merx/src/cart.php does not comply with psr-4 autoloading standard. Skipping.
Class Wagnerwagner\Merx\Gateways located in ./site/plugins/merx/src/gateways.php does not comply with psr-4 autoloading standard. Skipping.
Class Wagnerwagner\Merx\Merx located in ./site/plugins/merx/src/merx.php does not comply with psr-4 autoloading standard. Skipping.
Class Wagnerwagner\Merx\Payment located in ./site/plugins/merx/src/payment.php does not comply with psr-4 autoloading standard. Skipping.
Class Wagnerwagner\Merx\ProductList located in ./site/plugins/merx/src/productList.php does not comply with psr-4 autoloading standard. Skipping.
In Composer 2.0 I get:
Here I found an upgrading guide https://getcomposer.org/upgrade/UPGRADE-2.0.md