Try to find a Link matching req

        Expects req, an InstallRequirement and upgrade, a boolean
        Returns a Link if found,
        Raises DistributionNotFound or BestVersionAlreadyInstalled otherwise

def find_requirement(self, req, upgrade, ignore_compatibility=False):
        # type: (InstallRequirement, bool, bool) -> Optional[Link]
        """Try to find a Link matching req

        Expects req, an InstallRequirement and upgrade, a boolean
        Returns a Link if found,
        Raises DistributionNotFound or BestVersionAlreadyInstalled otherwise
        """
        all_candidates = self.find_all_candidates(req.name)

        # Filter out anything which doesn't match our specifier
        compatible_versions = set(
            req.specifier.filter(
                # We turn the version object into a str here because otherwise
                # when we're debundled but setuptools isn't, Python will see
                # packaging.version.Version and
                # pkg_resources._vendor.packaging.version.Version as different
                # types. This way we'll use a str as a common data interchange
                # format. If we stop using the pkg_resources provided specifier
                # and start using our own, we can drop the cast to str().
                [str(c.version) for c in all_candidates],
                prereleases=(
                    self.allow_all_prereleases
                    if self.allow_all_prereleases else None
                ),
            )
        )
        applicable_candidates = [
            # Again, converting to str to deal with debundling.
            c for c in all_candidates if str(c.version) in compatible_versions
        ]

        if applicable_candidates:
            best_candidate = max(applicable_candidates,
                                 key=self._candidate_sort_key)
        else:
            best_candidate = None

        if req.satisfied_by is not None:
            installed_version = parse_version(req.satisfied_by.version)
        else:
            installed_version = None

        if installed_version is None and best_candidate is None:
            logger.critical(
                'Could not find a version that satisfies the requirement %s '
                '(from versions: %s)',
                req,
                ', '.join(
                    sorted(
                        {str(c.version) for c in all_candidates},
                        key=parse_version,
                    )
                )
            )

            raise DistributionNotFound(
                'No matching distribution found for %s' % req
            )

        best_installed = False
        if installed_version and (
                best_candidate is None or
                best_candidate.version <= installed_version):
            best_installed = True

        if not upgrade and installed_version is not None:
            if best_installed:
                logger.debug(
                    'Existing installed version (%s) is most up-to-date and '
                    'satisfies requirement',
                    installed_version,
                )
            else:
                logger.debug(
                    'Existing installed version (%s) satisfies requirement '
                    '(most up-to-date version is %s)',
                    installed_version,
                    best_candidate.version,
                )
            return None

        if best_installed:
            # We have an existing version, and its the best version
            logger.debug(
                'Installed version (%s) is most up-to-date (past versions: '
                '%s)',
                installed_version,
                ', '.join(sorted(compatible_versions, key=parse_version)) or
                "none",
            )
            raise BestVersionAlreadyInstalled

        logger.debug(
            'Using version %s (newest of versions: %s)',
            best_candidate.version,
            ', '.join(sorted(compatible_versions, key=parse_version))
        )
        return best_candidate.location