Allow conflicting extras in explicit index assignments

[charliermarsh]

Summary

This PR enables something like the "final boss" of PyTorch setups -- explicit support for CPU vs. GPU-enabled variants via extras:

[project]
name = "project"
version = "0.1.0"
requires-python = ">=3.13.0"
dependencies = []

[project.optional-dependencies]
cpu = [
    "torch==2.5.1+cpu",
]
gpu = [
    "torch==2.5.1",
]

[tool.uv.sources]
torch = [
    { index = "torch-cpu", extra = "cpu" },
    { index = "torch-gpu", extra = "gpu" },
]

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

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

[tool.uv]
conflicts = [
    [
        { extra = "cpu" },
        { extra = "gpu" },
    ],
]

It builds atop the conflicting extras work to allow sources to be marked as specific to a dedicated extra being enabled or disabled.

As part of this work, sources now have an extra field. If a source has an extra, it means that the source is only applied to the requirement when defined within that optional group. For example, { index = "torch-cpu", extra = "cpu" } above only applies to "torch==2.5.1+cpu".

The extra field does not mean that the source is "enabled" when the extra is activated. For example, this wouldn't work:

[project]
name = "project"
version = "0.1.0"
requires-python = ">=3.13.0"
dependencies = ["torch"]

[tool.uv.sources]
torch = [
    { index = "torch-cpu", extra = "cpu" },
    { index = "torch-gpu", extra = "gpu" },
]

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

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

In this case, the sources would effectively be ignored. Extras are really confusing... but I think this is correct? We don't want enabling or disabling extras to affect resolution information that's outside of the relevant optional group.

[charliermarsh]

This needs documentation, but I'm gonna wait for approval.