@return [String] the name of the source of ‘locked’ dependencies, i.e.
those passed to {Resolver#resolve} directly as the `base`
(see Gem::Resolver::Molinillo::SpecificationProvider#name_for_locking_dependency_source)
Gem::Resolver::Molinillo::SpecificationProvider#name_for_locking_dependency_source
Ensures any raised {NoSuchDependencyError} has its {NoSuchDependencyError#required_by} set. @yield