Add index URLs when provided via uv add --index or --default-index

[charliermarsh]

Summary

The behavior is as follows:

• If you provide --index or --default-index on the command-line, we add those indexes to the pyproject.toml (with names, if provided, as in --index pytorch=https://download.pytorch.org/whl/cu121.
• If you provide --index-url or --default-index, we warn, but don't add the indexes to the file. (This seems wrong -- why not add them?)
• If you provide an index with a name or URL that already exists, we remove that entry, and add the new index to the top of the list (since it now has highest priority).
• If you provide a --default-index, and an index already has default = true, we remove that entry, since it won't be used anymore.

We do not pin packages to specific indexes yet.