Allow multiple pinned indexes in tool.uv.sources (#7769)

## Summary

This PR lifts the restriction that a package must come from a single
index. For example, you can now do:

```toml
[project]
name = "project"
version = "0.1.0"
readme = "README.md"
requires-python = ">=3.12"
dependencies = ["jinja2"]

[tool.uv.sources]
jinja2 = [
    { index = "torch-cu118", marker = "sys_platform == 'darwin'"},
    { index = "torch-cu124", marker = "sys_platform != 'darwin'"},
]

[[tool.uv.index]]
name = "torch-cu118"
url = "https://download.pytorch.org/whl/cu118"

[[tool.uv.index]]
name = "torch-cu124"
url = "https://download.pytorch.org/whl/cu124"
```

The construction is very similar to the way we handle URLs today: you
can have multiple URLs for a given package, but they must appear in
disjoint forks. So most of the code is just adding that abstraction to
the resolver, following our handling of URLs.

Closes #7761.

crates/uv-resolver/src/lib.rs

@@ -34,6 +34,7 @@ mod error;
 mod exclude_newer;
 mod exclusions;
 mod flat_index;
+mod fork_indexes;
 mod fork_urls;
 mod graph_ops;
 mod lock;