Add superclass for DependencyResolver specs

This moves common accessor up to a common place, reducing retyping
required.
Add DependencyResolver::SpecSpecification

This is the superclass for VendorSpecification and IndexSpecification as
each shares most of their implementations.
Remove existing accessors

These were missed during the first refactor to Specification
Document DependencyResolver::Specification classes

Woo, 100%!