Drop use of Moose roles in X.

Role::HasMessage, Role::Identifiable::HasIdent, and
Role::Identifiable::HasTags were that only classes I was using that loaded
Moose. Since their only purpose here was to define a couple of attributes, we
can just define those attributes (except tags, which were never used anyway!),
instead, and drop the dependency on them. Their elimination reduces the
runtime of `t/sqitch` from around 38ms to 23ms. No part of Moose or Class::MOP
loads anymore. This is a significant win!