Add a universal resolution mode to pip compile (#4505)

## Summary

This needs more tests and a few more changes, but checkpointing it for
now.

crates/uv-resolver/src/python_requirement.rs

@@ -1,5 +1,5 @@
-use pep440_rs::VersionSpecifiers;
-use pep508_rs::StringVersion;
+use pep440_rs::{Operator, Version, VersionSpecifier, VersionSpecifiers};
+use pep508_rs::{MarkerExpression, MarkerTree, MarkerValueVersion, StringVersion};
 use uv_toolchain::{Interpreter, PythonVersion};
 
 use crate::RequiresPython;
@@ -59,10 +59,32 @@ impl PythonRequirement {
         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())
+    /// Return a [`MarkerTree`] representing the Python requirement.
+    ///
+    /// See: [`RequiresPython::to_marker_tree`]
+    pub fn to_marker_tree(&self) -> MarkerTree {
+        let version = match &self.target {
+            None => self.installed.version.clone(),
+            Some(PythonTarget::Version(version)) => version.version.clone(),
+            Some(PythonTarget::RequiresPython(requires_python)) => {
+                return requires_python.to_marker_tree()
+            }
+        };
+
+        let version_major_minor_only = Version::new(version.release().iter().take(2));
+        let expr_python_version = MarkerExpression::Version {
+            key: MarkerValueVersion::PythonVersion,
+            specifier: VersionSpecifier::from_version(Operator::Equal, version_major_minor_only)
+                .unwrap(),
+        };
+        let expr_python_full_version = MarkerExpression::Version {
+            key: MarkerValueVersion::PythonFullVersion,
+            specifier: VersionSpecifier::from_version(Operator::Equal, version).unwrap(),
+        };
+        MarkerTree::And(vec![
+            MarkerTree::Expression(expr_python_version),
+            MarkerTree::Expression(expr_python_full_version),
+        ])
     }
 }