Metasploit::Model::Module::Ancestor#contents handler_module

[#56005352]

In order to turn a single payload Module into a Class, a handler_module
is required.  In metasploit-framework this is defined by
Msf::Payload::Handled::ClassMethods#handler_module, but
Metasploit::Model::Module::Ancestor#contents from the factory will just
setup class methods to return a memoized handler Module.