[red-knot] Use Unknown | T_inferred for undeclared public symbols (#15674)

## Summary

Use `Unknown | T_inferred` as the type for *undeclared* public symbols.

## Test Plan

- Updated existing tests
- New test for external `__slots__` modifications.
- New tests for external modifications of public symbols.

crates/red_knot_python_semantic/src/symbol.rs

@@ -85,6 +85,14 @@ impl<'db> Symbol<'db> {
             Symbol::Unbound => self,
         }
     }
+
+    #[must_use]
+    pub(crate) fn map_type(self, f: impl FnOnce(Type<'db>) -> Type<'db>) -> Symbol<'db> {
+        match self {
+            Symbol::Type(ty, boundness) => Symbol::Type(f(ty), boundness),
+            Symbol::Unbound => Symbol::Unbound,
+        }
+    }
 }
 
 #[cfg(test)]