Defines an alias flag for an existing one.

name str, the flag name.
original_name str, the original flag name.
flag_values FlagValues, the FlagValues instance with which the flag will be registered. This should almost never need to be overridden.
module_name A string, the name of the module that defines this flag.

a handle to defined flag.

flags.FlagError UnrecognizedFlagError: if the referenced flag doesn't exist. DuplicateFlagError: if the alias name has been used by some existing flag.