Use Composer modules and convert modules to PSR-4

PSR-4 is much more robust and predictable. But to do this,
a source root dir must be specified for every module.
This could be done in the root file, but this is not very modular.
Instead, now every module declares its own source root by using
the amazing Composer Merge Plugin. This approach allows each module
to also declare its own dependencies. Together, these changes allow
modules to be easily extractable to separate pacakges when the need
arises, and in general improves modularity significantly.

modules/ppcp-api-client/src/ApiModule.php

@@ -0,0 +1,56 @@
+<?php
+/**
+ * The API module.
+ *
+ * @package WooCommerce\PayPalCommerce\ApiClient
+ */
+
+declare(strict_types=1);
+
+namespace WooCommerce\PayPalCommerce\ApiClient;
+
+use Dhii\Container\ServiceProvider;
+use Dhii\Modular\Module\ModuleInterface;
+use Interop\Container\ServiceProviderInterface;
+use Psr\Container\ContainerInterface;
+
+/**
+ * Class ApiModule
+ */
+class ApiModule implements ModuleInterface {
+
+	/**
+	 * Sets up the module.
+	 *
+	 * @return ServiceProviderInterface
+	 */
+	public function setup(): ServiceProviderInterface {
+		return new ServiceProvider(
+			require __DIR__ . '/../services.php',
+			require __DIR__ . '/../extensions.php'
+		);
+	}
+
+	/**
+	 * Runs the module.
+	 *
+	 * @param ContainerInterface $container The container.
+	 */
+	public function run( ContainerInterface $container ): void {
+		add_action(
+			'woocommerce_after_calculate_totals',
+			function ( \WC_Cart $cart ) {
+				$fees = $cart->fees_api()->get_fees();
+				WC()->session->set( 'ppcp_fees', $fees );
+			}
+		);
+	}
+
+	/**
+	 * Returns the key for the module.
+	 *
+	 * @return string|void
+	 */
+	public function getKey() {
+	}
+}