os.rename fails across mounted devices

[Curtis Hovey]

While investing a test failure on my local system, I discovered there were a few calls to os.rename.
    OSError: [Errno 18] Invalid cross-device link

This error could be a problem because we often rely on extra mounts to gain extra space. As the two offenders are involved with package building and streams, this is a risk we can mitigate with shutil.move.

[Curtis Hovey (sinzui)]

Aaron suggested we refactor so that the rename is not needed:
   we should get rid of the os.rename, and just update StanzaWriter.filename before writing. os.rename was atomic, and if we're getting rid of that, I don't see why we want to rename into place at all.