renderWidget / Use hookName as template source

module.php

<?php
/*
* [...]
*/

use PrestaShop\PrestaShop\Adapter\Cart\CartPresenter;

if (!defined('_PS_VERSION_'))
	exit;

use PrestaShop\PrestaShop\Core\Module\WidgetInterface;

class Ps_Shoppingcart extends Module implements WidgetInterface
{
	[...]

	public function getWidgetVariables($hookName, array $params)
	{
		[...]
	}

	public function renderWidget($hookName, array $params)
	{
		if (Configuration::isCatalogMode()) {
		    return;
		}

		$this->smarty->assign($this->getWidgetVariables($hookName, $params));

		$tplName = $hookName.'.tpl';
		$template_path = $this->getTemplatePath($tplName);
		if ($template_path !== null) {
			return $this->fetch('module:ps_shoppingcart/'.$tplName);
		}

		return $this->fetch('module:ps_shoppingcart/ps_shoppingcart.tpl');
	}
}

$this->getTemplatePath($tplName); was kept for backward compatibility. It's shouldn't be used with Smarty module resources ($this->fetch('module:ps_shoppingcart/xxx)) beacuse it will give you a full path.

You should implement your own method getModuleTemplatePath which check theme override, parent theme override and then module template :(

See https://github.com/PrestaShop/PrestaShop/blob/develop/classes/module/Module.php#L2306