Don't use PHPUnit's autoloader when the composer binary is used.

This is related to [a comment](https://github.com/sebastianbergmann/phpunit/commit/288bff7b0a08e99fc513e45cc65bfc7430e0659b#commitcomment-2723865) from 288bff7.

Some 3rd-party libraries (especially those providing custom test cases, printers, etc.) include calls to `require_once('PHPUnit/Autoload.php');`. This is obviously an issue if PHPUnit is installed via composer. This PR fixes that problem and provides a useful mechanism for library developers to accommodate for composer-based PHPUnit installs in the future.