perf: remove more redundant scanning

Doing the stronger version of pubgrub-rs#175 because it turns out to be important to cargo crates.

src/internal/core.rs

@@ -131,7 +131,7 @@ impl<DP: DependencyProvider> State<DP> {
                         // but so does checking for duplicates.
                         // In practice the most common pathology is adding the same package repeatedly.
                         // So we only check if it is duplicated with the last item.
-                        if self.unit_propagation_buffer.last() != Some(&package_almost) {
+                        if !self.unit_propagation_buffer.contains(&package_almost) {
                             self.unit_propagation_buffer.push(package_almost.clone());
                         }
                         // Add (not term) to the partial solution with incompat as cause.