add conflict markers to the lock file (#9370)

This PR adds a notion of "conflict markers" to the lock file as an
attempt to address #9289. The idea is to encode a new kind of boolean
expression indicating how to choose dependencies based on which extras
are activated.

As an example of what conflict markers look like, consider one of the
cases
brought up in #9289, where `anyio` had unconditional dependencies on
two different versions of `idna`. Now, those are gated by markers, like
this:

```toml
        [[package]]
        name = "anyio"
        version = "4.3.0"
        source = { registry = "https://pypi.org/simple" }
        dependencies = [
            { name = "idna", version = "3.5", source = { registry = "https://pypi.org/simple" }, marker = "extra == 'extra-7-project-foo'" },
            { name = "idna", version = "3.6", source = { registry = "https://pypi.org/simple" }, marker = "extra == 'extra-7-project-bar' or extra != 'extra-7-project-foo'" },
            { name = "sniffio" },
        ]
```

The odd extra values like `extra-7-project-foo` are an encoding of not
just the conflicting extra (`foo`) but also the package it's declared
for (`project`). We need both bits of information because different
packages may have the same extra name, even if they are completely
unrelated. The `extra-` part is a prefix to distinguish it from groups
(which, in this case, would be encoded as `group-7-project-foo` if `foo`
were a dependency group). And the `7` part indicates the length of the
package name which makes it possible to parse out the package and extra
name from this encoding. (We don't actually utilize that property, but
it seems like good sense to do it in case we do need to extra
information from these markers.)

While this preserves PEP 508 compatibility at a surface level, it does
require utilizing this encoding scheme in order
to evaluate them when they're present (which only occurs when
conflicting extras/groups are declared).

My sense is that the most complex part of this change is not just adding
conflict markers, but their simplification. I tried to address this in
the code comments and commit messages.

Reviewers should look at this commit-by-commit.

Fixes #9289, Fixes #9546, Fixes #9640, Fixes #9622, Fixes #9498, Fixes
#9701, Fixes #9734

crates/uv-resolver/src/lib.rs

@@ -22,7 +22,7 @@ pub use resolver::{
     PackageVersionsResult, Reporter as ResolverReporter, Resolver, ResolverEnvironment,
     ResolverProvider, VersionsResponse, WheelMetadataResult,
 };
-pub use universal_marker::UniversalMarker;
+pub use universal_marker::{ConflictMarker, UniversalMarker};
 pub use version_map::VersionMap;
 pub use yanks::AllowedYanks;