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-button/src/Helper/ThreeDSecure.php

@@ -0,0 +1,104 @@
+<?php
+/**
+ * Helper class to determine how to proceed with an order depending on the 3d secure feedback.
+ *
+ * @package WooCommerce\PayPalCommerce\Button\Helper
+ */
+
+declare(strict_types=1);
+
+namespace WooCommerce\PayPalCommerce\Button\Helper;
+
+use WooCommerce\PayPalCommerce\ApiClient\Entity\CardAuthenticationResult as AuthResult;
+use WooCommerce\PayPalCommerce\ApiClient\Entity\Order;
+
+/**
+ * Class ThreeDSecure
+ */
+class ThreeDSecure {
+
+
+	const NO_DECISION = 0;
+	const PROCCEED    = 1;
+	const REJECT      = 2;
+	const RETRY       = 3;
+
+	/**
+	 * Determine, how we proceed with a given order.
+	 *
+	 * @link https://developer.paypal.com/docs/business/checkout/add-capabilities/3d-secure/#authenticationresult
+	 *
+	 * @param Order $order The order for which the decission is needed.
+	 *
+	 * @return int
+	 */
+	public function proceed_with_order( Order $order ): int {
+		if ( ! $order->payment_source() ) {
+			return self::NO_DECISION;
+		}
+		if ( ! $order->payment_source()->card() ) {
+			return self::NO_DECISION;
+		}
+		if ( ! $order->payment_source()->card()->authentication_result() ) {
+			return self::NO_DECISION;
+		}
+		$result = $order->payment_source()->card()->authentication_result();
+		if ( $result->liability_shift() === AuthResult::LIABILITY_SHIFT_POSSIBLE ) {
+			return self::PROCCEED;
+		}
+
+		if ( $result->liability_shift() === AuthResult::LIABILITY_SHIFT_UNKNOWN ) {
+			return self::RETRY;
+		}
+		if ( $result->liability_shift() === AuthResult::LIABILITY_SHIFT_NO ) {
+			return $this->no_liability_shift( $result );
+		}
+		return self::NO_DECISION;
+	}
+
+	/**
+	 * Determines how to proceed depending on the Liability Shift.
+	 *
+	 * @param AuthResult $result The AuthResult object based on which we make the decision.
+	 *
+	 * @return int
+	 */
+	private function no_liability_shift( AuthResult $result ): int {
+
+		if (
+			$result->enrollment_status() === AuthResult::ENROLLMENT_STATUS_BYPASS
+			&& ! $result->authentication_result()
+		) {
+			return self::PROCCEED;
+		}
+		if (
+			$result->enrollment_status() === AuthResult::ENROLLMENT_STATUS_UNAVAILABLE
+			&& ! $result->authentication_result()
+		) {
+			return self::PROCCEED;
+		}
+		if (
+			$result->enrollment_status() === AuthResult::ENROLLMENT_STATUS_NO
+			&& ! $result->authentication_result()
+		) {
+			return self::PROCCEED;
+		}
+
+		if ( $result->authentication_result() === AuthResult::AUTHENTICATION_RESULT_REJECTED ) {
+			return self::REJECT;
+		}
+
+		if ( $result->authentication_result() === AuthResult::AUTHENTICATION_RESULT_NO ) {
+			return self::REJECT;
+		}
+
+		if ( $result->authentication_result() === AuthResult::AUTHENTICATION_RESULT_UNABLE ) {
+			return self::RETRY;
+		}
+
+		if ( ! $result->authentication_result() ) {
+			return self::RETRY;
+		}
+		return self::NO_DECISION;
+	}
+}