Moodle 4.2+ external API test error

[bwalkerl]

Likely a result of the same change in externallib in 4.2 mentioned in #796 and #803. I think we've mostly addressed the other issue when we added support for 4.2 but missed this test.

1) core_external\external_api_test::test_all_external_info with data set "tool_dataflows_trigger_dataflow" (stdClass Object (...))
Error: Class "external_api" not found

/var/www/moodle-44/admin/tool/dataflows/classes/external/trigger_dataflow.php:34
/var/www/moodle-44/lib/classes/component.php:155
/var/www/moodle-44/lib/external/classes/external_api.php:62
/var/www/moodle-44/lib/external/tests/external_api_test.php:400
/var/www/moodle-44/lib/phpunit/classes/advanced_testcase.php:72