Given that we're using the last segment of the directory name to identify legacy (pre-package.json) extensions for the purposes of updating, it seems like we should use that everywhere, rather than the full path. The only issue I could see with that is that someone could have the same extension installed in user and dev (which is not a good thing anyhow! AFAIK, we don't make any guarantees about the behavior when an extension is installed in two directories).

So, I *could* pass along the directory that the extension was found in, but it seems cleaner and more consistent to identify extensions by the last part of the directory path rather than the full path.