packagedcode.about.AboutPackage.get_package_root() should not depend on access to ABOUT file

I am running some code that calls the `get_package_root()` method on Package objects created from a VirtualCodebase and my program crashes because it calls `get_package_root()` on an `AboutPackage`. The reason it fails is because it attempts to open the about file the AboutPackage is for. (https://github.com/nexB/scancode-toolkit/blob/develop/src/packagedcode/about.py#L63) We should go ahead and perform the FIXME stated in the code.