This changes the packaging tool to only set the remote URL instead of recreating the remote.

Recreating it was removing some metadata that identified the repo as having been cloned from the "real" repo, so flutter --version would report an unknown channel and source. This just sets the URL so that it looks like it came from GitHub.

I also fixed some incorrect comments, and made unzip work on platforms other than Windows (even though we don't really need it there yet).

Fixes #15518