Package cmdflag handles flag processing common to several go tools.
ErrFlagTerminator indicates the distinguished token "--", which causes the flag package to treat all subsequent arguments as non-flags.
ParseOne sees if args is present in the given flag set and if so, sets its value and returns the flag along with the remaining (unused) arguments.
ParseOne always returns either a non-nil Flag or a non-nil error, and always consumes at least one argument (even on error).
Unlike (*flag.FlagSet).Parse, ParseOne does not log its own errors.
A FlagNotDefinedError indicates a flag-like argument that does not correspond to any registered flag in a FlagSet.
A NonFlagError indicates an argument that is not a syntactically-valid flag.