Respect named `--index` and `--default-index` values in `tool.uv.sources` (#7910)
## Summary
If you pass a named index via the CLI, you can now reference it as a
named source. This required some surprisingly large refactors, since we
now need to be able to track whether a given index was provided on the
CLI vs. elsewhere (since, e.g., we don't want users to be able to
reference named indexes defined in global configuration).
Closes https://github.com/astral-sh/uv/issues/7899.