Implement API for Premium plugin installation and activation

[MAILPOET-2431]

tasks/phpstan/bootstrap.php

@@ -16,3 +16,38 @@ if (!class_exists(WooCommerce::class)) {
 }
 
 require_once __DIR__ . '/function-stubs.php';
+
+// methods & classes for Premium plugin installation are required
+// only when needed so we need to let PHPStan know about them
+if (!function_exists('plugins_api')) {
+  /**
+   * @param string $action
+   * @param array|object $args
+   * @return object|array|WP_Error
+   */
+  function plugins_api($action, $args) {
+    return [];
+  }
+}
+
+if (!class_exists(WP_Ajax_Upgrader_Skin::class)) {
+  // phpcs:ignore
+  class WP_Ajax_Upgrader_Skin {}
+}
+
+if (!class_exists(Plugin_Upgrader::class)) {
+  // phpcs:ignore
+  class Plugin_Upgrader {
+    public function __construct($skin = null) {
+    }
+
+    /**
+     * @param string $package
+     * @param array $args
+     * @return bool|WP_Error
+     */
+    public function install($package, $args = []) {
+      return true;
+    }
+  }
+}