Remove dependency on \Composer\Autoload\ClassLoader

[phpfui]

If you are not using Composer for autoloading (because it is slow and bloated, see WFA), then the following code produces an error:

$autoloadFile = (new \ReflectionClass(\Composer\Autoload\ClassLoader::class))->getFileName();

The solution is to use class_exits first and skip the above execution.

[zbateson]

Thanks @phpfui ! The code around there for plugin discovery needed some care anyway, so I fixed it a little differently in a commit and released as 4.0.1 :)