Respect `--index-url` provided via requirements.txt (#1719)
## Summary
When we read `--index-url` from a `requirements.txt`, we attempt to
respect the `--index-url` provided by the CLI if it exists.
Unfortunately, `--index-url` from the CLI has a default value... so we
_never_ respect the `--index-url` in the requirements file.
This PR modifies the CLI to use `None`, and moves the default into logic
in the `IndexLocations `struct.
Closes https://github.com/astral-sh/uv/issues/1692.
