Prepare a requirement that would be obtained from req.link

def prepare_linked_requirement(
        self,
        req,  # type: InstallRequirement
        session,  # type: PipSession
        finder,  # type: PackageFinder
        upgrade_allowed,  # type: bool
        require_hashes  # type: bool
    ):
        # type: (...) -> DistAbstraction
        """Prepare a requirement that would be obtained from req.link
        """
        # TODO: Breakup into smaller functions
        if req.link and req.link.scheme == 'file':
            path = url_to_path(req.link.url)
            logger.info('Processing %s', display_path(path))
        else:
            logger.info('Collecting %s', req)

        with indent_log():
            # @@ if filesystem packages are not marked
            # editable in a req, a non deterministic error
            # occurs when the script attempts to unpack the
            # build directory
            req.ensure_has_source_dir(self.build_dir)
            # If a checkout exists, it's unwise to keep going.  version
            # inconsistencies are logged later, but do not fail the
            # installation.
            # FIXME: this won't upgrade when there's an existing
            # package unpacked in `req.source_dir`
            # package unpacked in `req.source_dir`
            if os.path.exists(os.path.join(req.source_dir, 'setup.py')):
                rmtree(req.source_dir)
            req.populate_link(finder, upgrade_allowed, require_hashes)

            # We can't hit this spot and have populate_link return None.
            # req.satisfied_by is None here (because we're
            # guarded) and upgrade has no impact except when satisfied_by
            # is not None.
            # Then inside find_requirement existing_applicable -> False
            # If no new versions are found, DistributionNotFound is raised,
            # otherwise a result is guaranteed.
            assert req.link
            link = req.link

            # Now that we have the real link, we can tell what kind of
            # requirements we have and raise some more informative errors
            # than otherwise. (For example, we can raise VcsHashUnsupported
            # for a VCS URL rather than HashMissing.)
            if require_hashes:
                # We could check these first 2 conditions inside
                # unpack_url and save repetition of conditions, but then
                # we would report less-useful error messages for
                # unhashable requirements, complaining that there's no
                # hash provided.
                if is_vcs_url(link):
                    raise VcsHashUnsupported()
                elif is_file_url(link) and is_dir_url(link):
                    raise DirectoryUrlHashUnsupported()
                if not req.original_link and not req.is_pinned:
                    # Unpinned packages are asking for trouble when a new
                    # version is uploaded. This isn't a security check, but
                    # it saves users a surprising hash mismatch in the
                    # future.
                    #
                    # file:/// URLs aren't pinnable, so don't complain
                    # about them not being pinned.
                    raise HashUnpinned()

            hashes = req.hashes(trust_internet=not require_hashes)
            if require_hashes and not hashes:
                # Known-good hashes are missing for this requirement, so
                # shim it with a facade object that will provoke hash
                # computation and then raise a HashMissing exception
                # showing the user what the hash should be.
                hashes = MissingHashes()

            try:
                download_dir = self.download_dir
                # We always delete unpacked sdists after pip ran.
                autodelete_unpacked = True
                if req.link.is_wheel and self.wheel_download_dir:
                    # when doing 'pip wheel` we download wheels to a
                    # dedicated dir.
                    download_dir = self.wheel_download_dir
                if req.link.is_wheel:
                    if download_dir:
                        # When downloading, we only unpack wheels to get
                        # metadata.
                        autodelete_unpacked = True
                    else:
                        # When installing a wheel, we use the unpacked
                        # wheel.
                        autodelete_unpacked = False
                unpack_url(
                    req.link, req.source_dir,
                    download_dir, autodelete_unpacked,
                    session=session, hashes=hashes,
                    progress_bar=self.progress_bar
                )
            except requests.HTTPError as exc:
                logger.critical(
                    'Could not install requirement %s because of error %s',
                    req,
                    exc,
                )
                raise InstallationError(
                    'Could not install requirement %s because of HTTP '
                    'error %s for URL %s' %
                    (req, exc, req.link)
                )
            abstract_dist = make_abstract_dist(req)
            with self.req_tracker.track(req):
                abstract_dist.prep_for_dist(finder, self.build_isolation)
            if self._download_should_save:
                # Make a .zip of the source_dir we already created.
                if req.link.scheme in vcs.all_schemes:
                    req.archive(self.download_dir)
        return abstract_dist