Install an egg into the egg directory

def install_egg(self, egg_name):
        """ Install an egg into the egg directory """
        if not os.path.exists(self.egg_directory):
            os.makedirs(self.egg_directory)
        self.requirement_set.add_requirement(
            InstallRequirement.from_line(egg_name, None))
        try:
            self.requirement_set.prepare_files(self.finder)
            self.requirement_set.install(['--prefix=' + self.egg_directory], [])
        except DistributionNotFound:
            self.requirement_set.requirements._keys.remove(egg_name)
            raise PipException()