function toOverwriteOptions(config) {
    const options = []

    for (const packageName of Object.keys(config)) {
        const packageConfig = config[packageName]

        for (const variableName of Object.keys(packageConfig)) {
            const value = packageConfig[variableName]

            options.push(`--${packageName}:${variableName}=${value}`)
        }
    }

    return options
}
Converts a given config object to an `--:=` style option array.

@param {object|null} config -
  A map-like object to overwrite package configs.
  Keys are package names.
  Every value is a map-like object (Pairs of variable name and value).
@returns {string[]} `--:=` style options.