Does not work with phpunit phar release

When PhpUnit is imported from the phar release, the class `SebastianBergmann\Exporter\Exporter` is vendored as `PhpUnit\SebastianBergmann\Exporter\Exporter`.

The `DefinitionHasArgumentConstraint` in this bundle has a property of type `SebastianBergmann\Exporter\Exporter` which cause tests to fail, because the autoloader can't find it.