## Summary
This PR revives https://github.com/astral-sh/uv/pull/10017, which might
be viable now that we _don't_ enforce any platforms by default.
The basic idea here is that users can mark certain platforms as required
(empty, by default). When resolving, we ensure that the specified
platforms have wheel coverage, backtracking if not.
For example, to require that we include a version of PyTorch that
supports Intel macOS:
```toml
[project]
name = "project"
version = "0.1.0"
requires-python = ">=3.11"
dependencies = ["torch>1.13"]
[tool.uv]
required-platforms = [
"sys_platform == 'darwin' and platform_machine == 'x86_64'"
]
```
Other than that, the forking is identical to past iterations of this PR.
This would give users a way to resolve the tail of issues in #9711, but
with manual opt-in to supporting specific platforms.
172305abb6