`requirements-txt-fixer` ordering breaks using --index-url and --extra-index-url together in requirements.txt

If you use both the --index-url and --extra-index-url flags together in a requirements.txt file the hook will put --extra-index-url above the --index-url which causes the url set to index-url be skipped. I suppose this is because the flags are also sorted alphabetically.

Start at the requirements-txt-fixer hook entry point and trace how --index-url and --extra-index-url are ordered in requirements.txt. Check the related tests if present, then verify that --index-url remains before --extra-index-url so both settings are honored.