## Summary
There's a good example of the downside of using verbatim URLs here:
https://github.com/astral-sh/uv/pull/14197#discussion_r2163599625 (we
show two relative paths that point to the same directory, but it's not
clear from the error message).
The diff:
```
2 2 │ ----- stdout -----
3 3 │
4 4 │ ----- stderr -----
5 5 │ error: Requirements contain conflicting URLs for package `library` in all marker environments:
6 │-- ../../library
7 │-- ./library
6 │+- file://[TEMP_DIR]/library
7 │+- file://[TEMP_DIR]/library (editable)
```
## Summary
This PR modifies the requirement source entities to store a (new)
container struct that wraps `IndexUrl`. This will allow us to store
user-defined metadata alongside `IndexUrl`, and propagate that metadata
throughout resolution.
Specifically, I need to store the "kind" of the index (Simple API vs.
`--find-links`), but I also ran into this problem when I tried to add
support for overriding `Cache-Control` headers on a per-index basis: at
present, we have no way to passing around metadata alongside an
`IndexUrl`.
This updates the surrounding code to use the new ResolverEnvironment
type. In some cases, this simplifies caller code by removing case
analysis. There *shouldn't* be any behavior changes here. Some test
snapshots were updated to account for some minor tweaks to error
messages.
I didn't split this up into separate commits because it would have been
too difficult/costly.
## 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.
Charlie Marsh
Andrew Gallant