uv-resolver: use Requires-Python to filter dependencies during universal resolution

In the time before universal resolving, we would always pass a
`MarkerEnvironment`, and this environment would capture any relevant
`Requires-Python` specifier (including if `-p/--python` was provided on
the CLI).

But in universal resolution, we very specifically do not use a
`MarkerEnvironment` because we want to produce a resolution that is
compatible across potentially multiple environments. This in turn meant
that we lost `Requires-Python` filtering.

This PR adds it back. We do this by converting our `PythonRequirement`
into a `MarkerTree` that encodes the version specifiers in a
`Requires-Python` specifier. We then ask whether that `MarkerTree` is
disjoint with a dependency specification's `MarkerTree`. If it is, then
we know it's impossible for that dependency specification to every be
true, and we can completely ignore it.

crates/uv-resolver/src/python_requirement.rs

@@ -58,6 +58,12 @@ impl PythonRequirement {
     pub fn target(&self) -> Option<&PythonTarget> {
         self.target.as_ref()
     }
+
+    /// Return the target version of Python as a "requires python" type,
+    /// if available.
+    pub(crate) fn requires_python(&self) -> Option<&RequiresPython> {
+        self.target().and_then(|target| target.as_requires_python())
+    }
 }
 
 #[derive(Debug, Clone, Eq, PartialEq)]