Search for the specifications that match the given dependency.

        The specifications in the returned list will be considered in reverse
        order, so the latest version ought to be last.

def search_for(self, dependency):  # type: (Dependency) -> List[Package]
        """
        Search for the specifications that match the given dependency.

        The specifications in the returned list will be considered in reverse
        order, so the latest version ought to be last.
        """
        if dependency.is_root:
            return PackageCollection(dependency, [self._package])

        for constraint in self._search_for.keys():
            if (
                constraint.name == dependency.name
                and constraint.constraint.intersect(dependency.constraint)
                == dependency.constraint
            ):
                packages = [
                    p
                    for p in self._search_for[constraint]
                    if dependency.constraint.allows(p.version)
                ]

                packages.sort(
                    key=lambda p: (
                        not p.is_prerelease() and not dependency.allows_prereleases(),
                        p.version,
                    ),
                    reverse=True,
                )

                return PackageCollection(dependency, packages)

        if dependency.is_vcs():
            packages = self.search_for_vcs(dependency)
        elif dependency.is_file():
            packages = self.search_for_file(dependency)
        elif dependency.is_directory():
            packages = self.search_for_directory(dependency)
        else:
            constraint = dependency.constraint

            packages = self._pool.find_packages(
                dependency.name,
                constraint,
                extras=dependency.extras,
                allow_prereleases=dependency.allows_prereleases(),
            )

            packages.sort(
                key=lambda p: (
                    not p.is_prerelease() and not dependency.allows_prereleases(),
                    p.version,
                ),
                reverse=True,
            )

        self._search_for[dependency] = packages

        return PackageCollection(dependency, packages)