Require stance only for auxiliary and exploit

[#49858419]
[SEERM #7958]

Mdm::Module::Detail#stance should only be validated for inclusion in
Mdm::Module::Detail::STANCES if Mdm::Module::Detail#mtype is 'auxiliary'
or 'exploit' because in metasploit-framework, only those types of
modules use stances.  Mdm::Module::Detail#stance being `nil` for other
types is not enforced in case there are incorrect modules out there: its
better that the cache work.