[red-knot] don't include Unknown in the type for a conditionally-defined import (#13563)

## Summary

Fixes the bug described in #13514 where an unbound public type defaulted
to the type or `Unknown`, whereas it should only be the type if unbound.

## Test Plan

Added a new test case

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

crates/red_knot_python_semantic/src/stdlib.rs

@@ -37,6 +37,13 @@ fn core_module_symbol_ty<'db>(
 ) -> Type<'db> {
     resolve_module(db, &core_module.name())
         .map(|module| global_symbol_ty(db, module.file(), symbol))
+        .map(|ty| {
+            if ty.is_unbound() {
+                ty
+            } else {
+                ty.replace_unbound_with(db, Type::Never)
+            }
+        })
         .unwrap_or(Type::Unbound)
 }