ext_tables.php

<?php

use TYPO3\CMS\Core\Utility\ExtensionManagementUtility;

defined('TYPO3') or die('Access denied.');

$isComposerMode = defined('TYPO3_COMPOSER_MODE') && TYPO3_COMPOSER_MODE;
if (!$isComposerMode) {
    // we load the autoloader for our libraries
    $dir = ExtensionManagementUtility::extPath('solr');
    require $dir . '/Resources/Private/Php/ComposerLibraries/vendor/autoload.php';
}