[red-knot] Minor improvements to KnownFunction API (#15441)

A small PR to reduce some of the code duplication between the various
branches, make it a little more readable and move the API closer to what
we already have for `KnownClass`

crates/red_knot_python_semantic/src/module_resolver/module.rs

@@ -154,6 +154,10 @@ impl KnownModule {
         }
     }
 
+    pub const fn is_builtins(self) -> bool {
+        matches!(self, Self::Builtins)
+    }
+
     pub const fn is_typing(self) -> bool {
         matches!(self, Self::Typing)
     }