Add State::add_version

This wrapper avoids accessing `incompatibility_store` directly in uv code.

Before:

```rust
let dep_incompats = self.pubgrub.add_incompatibility_from_dependencies(
    package.clone(),
    version.clone(),
    dependencies,
);
self.pubgrub.partial_solution.add_version(
    package.clone(),
    version.clone(),
    dep_incompats,
    &self.pubgrub.incompatibility_store,
);
```

After:

```rust
self.pubgrub.add_version(package.clone(), version.clone(), dependencies);
```

src/internal/core.rs

@@ -74,6 +74,26 @@ impl<DP: DependencyProvider> State<DP> {
         }
     }
 
+    /// Make a decision for the current package.
+    pub fn add_version(
+        &mut self,
+        package: DP::P,
+        version: DP::V,
+        dependencies: impl IntoIterator<Item = (DP::P, DP::VS)>,
+    ) {
+        let dep_incompats = self.add_incompatibility_from_dependencies(
+            package.clone(),
+            version.clone(),
+            dependencies,
+        );
+        self.partial_solution.add_version(
+            package.clone(),
+            version.clone(),
+            dep_incompats,
+            &self.incompatibility_store,
+        )
+    }
+
     /// Add an incompatibility to the state.
     pub fn add_incompatibility(&mut self, incompat: Incompatibility<DP::P, DP::VS, DP::M>) {
         let id = self.incompatibility_store.alloc(incompat);