The root directory discovery is convoluted, and there are cases where it fails to resolve to a suitable path. We should improve the discovery as much as possible, but it also makes sense to provide a way for users to explicitly specify the root directory.

We could add a --rootdir command line option, which would be relative to the invocation directory. We could also add a rootdir configuration key, which would override the command line option, and would be relative to the configuration file.