Resolve what operations need to be done

        As a side-effect of this method, the packages (and their dependencies)
        are downloaded, unpacked and prepared for installation. This
        preparation is done by ``pip.operations.prepare``.

        Once PyPI has static dependency metadata available, it would be
        possible to move the preparation to become a step separated from
        dependency resolution.

def resolve(self, requirement_set):
        # type: (RequirementSet) -> None
        """Resolve what operations need to be done

        As a side-effect of this method, the packages (and their dependencies)
        are downloaded, unpacked and prepared for installation. This
        preparation is done by ``pip.operations.prepare``.

        Once PyPI has static dependency metadata available, it would be
        possible to move the preparation to become a step separated from
        dependency resolution.
        """
        # make the wheelhouse
        if self.preparer.wheel_download_dir:
            ensure_dir(self.preparer.wheel_download_dir)

        # If any top-level requirement has a hash specified, enter
        # hash-checking mode, which requires hashes from all.
        root_reqs = (
            requirement_set.unnamed_requirements +
            list(requirement_set.requirements.values())
        )
        self.require_hashes = (
            requirement_set.require_hashes or
            any(req.has_hash_options for req in root_reqs)
        )

        # Display where finder is looking for packages
        locations = self.finder.get_formatted_locations()
        if locations:
            logger.info(locations)

        # Actually prepare the files, and collect any exceptions. Most hash
        # exceptions cannot be checked ahead of time, because
        # req.populate_link() needs to be called before we can make decisions
        # based on link type.
        discovered_reqs = []  # type: List[InstallRequirement]
        hash_errors = HashErrors()
        for req in chain(root_reqs, discovered_reqs):
            try:
                discovered_reqs.extend(
                    self._resolve_one(requirement_set, req)
                )
            except HashError as exc:
                exc.req = req
                hash_errors.append(exc)

        if hash_errors:
            raise hash_errors