(internal) Download an ``url`` to a ``target``.

def download_file(self, url, target, cwd=None):
        """
        (internal) Download an ``url`` to a ``target``.
        """
        if not url:
            return
        info('Downloading {} from {}'.format(self.name, url))

        if cwd:
            target = join(cwd, target)

        parsed_url = urlparse(url)
        if parsed_url.scheme in ('http', 'https'):
            def report_hook(index, blksize, size):
                if size <= 0:
                    progression = '{0} bytes'.format(index * blksize)
                else:
                    progression = '{0:.2f}%'.format(
                        index * blksize * 100. / float(size))
                if "CI" not in environ:
                    stdout.write('- Download {}\r'.format(progression))
                    stdout.flush()

            if exists(target):
                unlink(target)

            # Download item with multiple attempts (for bad connections):
            attempts = 0
            while True:
                try:
                    urlretrieve(url, target, report_hook)
                except OSError as e:
                    attempts += 1
                    if attempts >= 5:
                        raise e
                    stdout.write('Download failed retrying in a second...')
                    time.sleep(1)
                    continue
                break
            return target
        elif parsed_url.scheme in ('git', 'git+file', 'git+ssh', 'git+http', 'git+https'):
            if isdir(target):
                with current_directory(target):
                    shprint(sh.git, 'fetch', '--tags')
                    if self.version:
                        shprint(sh.git, 'checkout', self.version)
                    shprint(sh.git, 'pull')
                    shprint(sh.git, 'pull', '--recurse-submodules')
                    shprint(sh.git, 'submodule', 'update', '--recursive')
            else:
                if url.startswith('git+'):
                    url = url[4:]
                shprint(sh.git, 'clone', '--recursive', url, target)
                if self.version:
                    with current_directory(target):
                        shprint(sh.git, 'checkout', self.version)
                        shprint(sh.git, 'submodule', 'update', '--recursive')
            return target