Mdm::Module namespace

[Metasploit Data Models](https://github.com/rapid7/metasploit_data_models) is being [updated](https://github.com/rapid7/metasploit_data_models/pull/18) so that `Mdm::Module` is a namespace instead of a prefix for all module metadata related models.

These commits first add specs around metasploit-framework's usage of the `Mdm::Module*` models and then update the code to use the new `Mdm::Module::*` names.