I think these changes cover the move from the previous functionality to the current, but I imagine you would need to implement 2 new test cases to cover parent directory and current directory sites/all/modules directories.  It's what I had to do with my package type pr changes.  Sorry to keep mentioning it, but I think you may be trying to achieve with drupal what I wanted to achieve with wordpress.  [Test Case](https://github.com/fancyguy/installers/blob/f2b4105ade20c3e62fa802886b3c33e9b0a42991/tests/Composer/Installers/Test/InstallerTest.php#L221)