dev/core#1615 - Fix for recently migrated civicrm-setup

dev/core#1615 - Fix for recently migrated civicrm-setup

Overview
--------

Fix a recent (intra-alpha) regression from https://github.com/civicrm/civicrm-core/pull/16618

The search for `civicrm-setup` goes slightly awry because the migrated copy of civicrm-setup uses
the core's classloader rather than its own classloader.

Steps to Reproduce
------------------

* Create a site with WP and Civi(master).
* Ensure Civi is uninstalled. (ex: `cv core:uninstall -f`)
* In `wp-content/plugins/civicrm/civicrm`, touch the file `.use-civicrm-setup`.
  (*This is supposed to activate the new UI.*)
* Go to the WP admin console. In the left navbar, choose "CiviCRM Installer".

Before
------

It fails to load the new UI -- the search finds `.use-civicrm-setup` but does not
find `civicrm-setup-autoload.php`. It falls back to the old UI.

After
-----

It opens the new UI.