Add support for `package`-level conflicts in workspaces (#14906)
Revives https://github.com/astral-sh/uv/pull/9130
Previously, we allowed scoping conflicting extras or groups to specific
packages, e.g. ,`{ package = "foo", extra = "bar" }` for a conflict in
`foo[bar]`. Now, we allow dropping the `extra` or `group` bit and using
`{ package = "foo" }` directly which declares a conflict with `foo`'s
production dependencies.
This means you can declare conflicts between workspace members, e.g.:
```
[tool.uv]
conflicts = [[{ package = "foo" }, { package = "bar" }]]
```
would not allow `foo` and `bar` to be installed at the same time.
Similarly, a conflict can be declared between a package and a group:
```
[tool.uv]
conflicts = [[{ package = "foo" }, { group = "lint" }]]
```
which would mean, e.g., that `--only-group lint` would be required for
the invocation.
As with our existing support for conflicting extras, there are
edge-cases here where the resolver will _not_ fail even if there are
conflicts that render a particular install target unusable. There's test
coverage for some of these. We'll still error at install-time when the
conflicting groups are selected. Due to the likelihood of bugs in this
feature, I've marked it as a preview feature.
I would not recommend reading the commits as there's some slop from not
wanting to rebase Andrew's branch.
---------
Co-authored-by: Andrew Gallant <andrew@astral.sh>
