Allow multiple pinned indexes in tool.uv.sources

[charliermarsh]

Summary

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

[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.

[charliermarsh]

This one actually fails:

[project]
name = "project"
version = "0.1.0"
requires-python = ">=3.12"
dependencies = ["jinja2>=3"]

[tool.uv.sources]
jinja2 = [
    { index = "torch-cu118", marker = "sys_platform == 'win32'"},
]

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

(It succeeds if you use explicit = true on the index.)

The issue is that the sys_platform == 'win32' branch uses the torch-cu118 index explicitly, but then the implied sys_platform != 'win32' branch also resolves to that same index.