So we have:
descriptor.filename: the path to the UFO, relative to the documentpath
descriptor.path: the resolved, absolute path to the UFO
This means we have to be aware of a couple of situations, described in updatePaths() and testPatgNameResolve().
Case 1: both filename and path attributes are None. Action: write the descriptor as is, without filename attr.
Case 2: filename attribute points somewhere, but the path attribute is None. So we can't actually verify if the UFO really exists, but we don't have to. Action: write the filename attribute as is. We could calculate a new path though.
Case 3: filename attribute is None, path attribute has a path. So there is no legacy value for filename that we need to look out for. Action: we can calculate a new relative path and store that in filename.
Case 4: filename and path attributes are not None, but they're in conflict, pointing to different places/ So the absolute path of the UFO and the absolute path of the document produce a different relative path than is stored in filename. One of them must be wrong.
When a new filename is set, make sure to set the path attribute to None and vice versa.
13abae94c5