fix: expose DefaultResolverProvider (#1764)

## Summary

The `DefaultResolverProvider` struct was not public. This PR exposes it
so we can build our own and use this as a fallback.

## Test Plan

I did not explicitly test this trivial change.

crates/uv-resolver/src/lib.rs

@@ -7,7 +7,8 @@ pub use prerelease_mode::PreReleaseMode;
 pub use resolution::{Diagnostic, DisplayResolutionGraph, ResolutionGraph};
 pub use resolution_mode::ResolutionMode;
 pub use resolver::{
-    BuildId, InMemoryIndex, Reporter as ResolverReporter, Resolver, ResolverProvider,
+    BuildId, DefaultResolverProvider, InMemoryIndex, Reporter as ResolverReporter, Resolver,
+    ResolverProvider,
 };
 
 mod candidate_selector;

crates/uv-resolver/src/resolver/mod.rs

@@ -46,7 +46,7 @@ use crate::python_requirement::PythonRequirement;
 use crate::resolution::ResolutionGraph;
 use crate::resolver::allowed_urls::AllowedUrls;
 pub use crate::resolver::index::InMemoryIndex;
-use crate::resolver::provider::DefaultResolverProvider;
+pub use crate::resolver::provider::DefaultResolverProvider;
 pub use crate::resolver::provider::ResolverProvider;
 pub(crate) use crate::resolver::provider::VersionsResponse;
 use crate::resolver::reporter::Facade;