factor out Pkg/DepIterator prettyprinters into own header

factor out Pkg/DepIterator prettyprinters into own header

The old prettyprinters have only access to the struct they pretty print,
which isn't enough usually as we want to know for a package also a bit
of state information like which version is the candidate.

We therefore need to pull the DepCache into context and hence use a
temporary struct which is printed instead of the iterator itself.