fix: expose types to implement custom ResolverProvider (#1862)

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## Summary

To integrate `uv` into `pixi` I need to specify a custom
`ResolverProvider` to be able to specify that some packages are already
installed by conda and should not be touched. However, some of the types
required to implement your own `ResolverProvider` were not accessible
through the public API. This PR basically adds them.

## Test Plan

I didnt add an explicit test for this.

crates/uv-resolver/src/lib.rs

@@ -4,12 +4,15 @@ pub use finder::{DistFinder, Reporter as FinderReporter};
 pub use manifest::Manifest;
 pub use options::{Options, OptionsBuilder};
 pub use prerelease_mode::PreReleaseMode;
+pub use python_requirement::PythonRequirement;
 pub use resolution::{AnnotationStyle, Diagnostic, DisplayResolutionGraph, ResolutionGraph};
 pub use resolution_mode::ResolutionMode;
 pub use resolver::{
-    BuildId, DefaultResolverProvider, InMemoryIndex, Reporter as ResolverReporter, Resolver,
-    ResolverProvider,
+    BuildId, DefaultResolverProvider, InMemoryIndex, PackageVersionsResult,
+    Reporter as ResolverReporter, Resolver, ResolverProvider, VersionsResponse,
+    WheelMetadataResult,
 };
+pub use version_map::VersionMap;
 
 mod candidate_selector;
 mod constraints;