Fix requirements-txt-fixer reordering --index-url after --extra-index-url (#612)

[sudorm-rf0]

Summary

Fixes #612 — requirements-txt-fixer sorts requirements alphabetically, which
reorders --index-url after --extra-index-url. That breaks pip: the primary
index URL is silently dropped because pip only honours the first
--index-url it sees.

Before

A requirements.txt written in the correct order:

--index-url https://primary.example.com
--extra-index-url https://extra.example.com
foo==1.0

was "fixed" (reordered) to:

--extra-index-url https://extra.example.com
--index-url https://primary.example.com
foo==1.0

With --extra-index-url first, pip drops the primary index URL.

After

--index-url is now always kept before --extra-index-url, matching pip's
expected semantics.

Root cause

In requirements_txt_fixer.py, each line (including --index-url and
--extra-index-url) is treated as a Requirement and sorted by name via
Requirement.__lt__. Since '--extra-index-url' < '--index-url'
alphabetically, the extra index ended up first.

Fix

Special-case the two options in Requirement.__lt__ so --index-url always
sorts before --extra-index-url. All other ordering (packages, comments,
duplicates) is unchanged.

Tests

Added two cases to tests/requirements_txt_fixer_test.py:

• --index-url already first → left unchanged (PASS).
• --extra-index-url first → reordered so --index-url comes first (FAIL,
  file rewritten).

Verification

• pytest — tests/requirements_txt_fixer_test.py: 33 passed (including the
  two new cases).
• pre-commit run --files ... — all hooks pass (flake8, mypy, pyupgrade,
  reorder-python-imports, add-trailing-comma, autopep8).