[ty] Favour imported symbols over builtin symbols (#21285)

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## Summary

Raised by @AlexWaygood.

We previously did not favour imported symbols, when we probably
should've

## Test Plan

Add test showing that we favour imported symbol even if it is
alphabetically after other symbols that are builtin.

crates/ty_completion_eval/completion-evaluation-tasks.csv

@@ -17,7 +17,7 @@ numpy-array,main.py,1,1
 object-attr-instance-methods,main.py,0,1
 object-attr-instance-methods,main.py,1,1
 raise-uses-base-exception,main.py,0,2
-scope-existing-over-new-import,main.py,0,13
+scope-existing-over-new-import,main.py,0,1
 scope-prioritize-closer,main.py,0,2
 scope-simple-long-identifier,main.py,0,1
 tstring-completions,main.py,0,1

crates/ty_ide/src/completion.rs

@@ -883,9 +883,10 @@ fn is_in_definition_place(db: &dyn Db, tokens: &[Token], file: File) -> bool {
 /// This has the effect of putting all dunder attributes after "normal"
 /// attributes, and all single-underscore attributes after dunder attributes.
 fn compare_suggestions(c1: &Completion, c2: &Completion) -> Ordering {
-    fn key<'a>(completion: &'a Completion) -> (bool, NameKind, bool, &'a Name) {
+    fn key<'a>(completion: &'a Completion) -> (bool, bool, NameKind, bool, &'a Name) {
         (
             completion.module_name.is_some(),
+            completion.builtin,
             NameKind::classify(&completion.name),
             completion.is_type_check_only,
             &completion.name,
@@ -4196,6 +4197,25 @@ type <CURSOR>
         ");
     }
 
+    #[test]
+    fn favour_imported_over_builtin() {
+        let snapshot =
+            completion_test_builder("from typing import Protocol\nclass Foo(P<CURSOR>: ...")
+                .filter(|c| c.name.starts_with('P'))
+                .build()
+                .snapshot();
+
+        // Here we favour `Protocol` over the other completions
+        // because `Protocol` has been imported, and the other completions are builtin.
+        assert_snapshot!(snapshot, @r"
+        Protocol
+        PendingDeprecationWarning
+        PermissionError
+        ProcessLookupError
+        PythonFinalizationError
+        ");
+    }
+
     /// A way to create a simple single-file (named `main.py`) completion test
     /// builder.
     ///