Add index URLs when provided via uv add --index or --default-index (#7746)

## 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.

crates/uv-cli/src/lib.rs

@@ -768,6 +768,10 @@ impl<T> Maybe<T> {
             Maybe::None => None,
         }
     }
+
+    pub fn is_some(&self) -> bool {
+        matches!(self, Maybe::Some(_))
+    }
 }
 
 /// Parse a string into an [`IndexUrl`], mapping the empty string to `None`.