Package::to_json_file is wrong by inspection

[doug-q]

It opens the file in read mode then writes to it. No users(or tests), but it's a landmine.

Locally I now have:

    /// Write the Package into a json file.
    pub fn to_json_file(&self, path: impl AsRef<Path>) -> Result<(), PackageEncodingError> {
        let file = fs::OpenOptions::new().write(true).truncate(true).create(true).open(path)?;
        let writer = io::BufWriter::new(file);
        self.to_json_writer(writer)
    }