uv-resolver: introduce new UniversalMarker type

This effectively combines a PEP 508 marker and an as-yet-specified
marker for expressing conflicts among extras and groups.

This just defines the type and threads it through most of the various
points in the code that previously used `MarkerTree` only. Some parts
do still continue to use `MarkerTree` specifically, e.g., when dealing
with non-universal resolution or exporting to `requirements.txt`.

This doesn't change any behavior.

crates/uv-resolver/src/lib.rs

@@ -22,6 +22,7 @@ pub use resolver::{
     PackageVersionsResult, Reporter as ResolverReporter, Resolver, ResolverEnvironment,
     ResolverProvider, VersionsResponse, WheelMetadataResult,
 };
+pub use universal_marker::UniversalMarker;
 pub use version_map::VersionMap;
 pub use yanks::AllowedYanks;
 
@@ -56,5 +57,6 @@ mod requires_python;
 mod resolution;
 mod resolution_mode;
 mod resolver;
+mod universal_marker;
 mod version_map;
 mod yanks;