fix #43668 PluginAdapter checkExistingExtension does not load plugin

[svanschu]

Pull Request for Issue #43668 .

Summary of Changes

Do the same extension load in the checkExistingExtension as for InstallerAdapter

Testing Instructions

Try to install a plugin as update and get the information of the current installed plugin in the preflight.

For example

public function preflight(string $type, InstallerAdapter $parent): bool
{
    $cache         = new Registry($parent->extension->manifest_cache);
    $this->oldVersion = $cache->get('version');

    return true;
}

Actual result BEFORE applying this Pull Request

$this->oldVersion = null

Expected result AFTER applying this Pull Request

$this->oldVersion = <the actual currently installed version>

Link to documentations

Please select:

• [ ] Documentation link for docs.joomla.org:

• [ ] No documentation changes for docs.joomla.org needed

• [ ] Pull Request link for manual.joomla.org:

• [ ] No documentation changes for manual.joomla.org needed