Handle universal vs. fork markers with ResolverMarkers (#5099)

* Use a dedicated `ResolverMarkers` check in the fork state. This is
better than the `MarkerTree::And(Vec::new())` check.
* Report the timing correct naming universal resolution instead of two
spaces around an empty string when there are no markers.
* On resolution error, show the split that we're in. I'm not sure how to
word this, since we're doing a universal resolution until we fork, so
the trace may contain information from requirements that are not part of
this fork.

crates/uv-dispatch/src/lib.rs

@@ -26,6 +26,7 @@ use uv_installer::{Installer, Plan, Planner, Preparer, SitePackages};
 use uv_python::{Interpreter, PythonEnvironment};
 use uv_resolver::{
     ExcludeNewer, FlatIndex, InMemoryIndex, Manifest, OptionsBuilder, PythonRequirement, Resolver,
+    ResolverMarkers,
 };
 use uv_types::{BuildContext, BuildIsolation, EmptyInstalledPackages, HashStrategy, InFlight};
 
@@ -146,7 +147,7 @@ impl<'a> BuildContext for BuildDispatch<'a> {
                 .index_strategy(self.index_strategy)
                 .build(),
             &python_requirement,
-            Some(markers),
+            ResolverMarkers::SpecificEnvironment(markers.clone()),
             Some(tags),
             self.flat_index,
             self.index,