Parameterize require_hashes within Resolver
Parameterize require_hashes within Resolver

Removes the only dynamic state from Resolver, at the cost of passing
an extra variable to 2 methods.

sbidoul/pip,sbidoul/pip,pypa/pip,xavfernandez/pip,pypa/pip,pfmoore/pip,xavfernandez/pip,pradyunsg/pip,xavfernandez/pip,pfmoore/pip,pradyunsg/pip