feat(tvix/eval): emit warnings on useless inherit

Change-Id: Ifb9993cf8b85393eb43e1b204c7ab2f889b7113b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6194
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/warnings.rs

@@ -4,6 +4,7 @@
 #[derive(Debug)]
 pub enum WarningKind {
     DeprecatedLiteralURL,
+    UselessInherit,
 }
 
 #[derive(Debug)]