compile: Allow object generating rules to be annotated as entrypoint (#5578)

Using ground ref of rule when generating entrypoint from annotation
as variable in head suffix cannot be included in generated entrypoint path.

Fixes: #5577
Signed-off-by: Johan Fylling <johan.dev@fylling.se>

compile/compile.go

@@ -234,7 +234,7 @@ func addEntrypointsFromAnnotations(c *Compiler, ar []*ast.AnnotationsRef) error
 				}
 			case "rule":
 				if r := ref.GetRule(); r != nil {
-					entrypoint = r.Ref()
+					entrypoint = r.Ref().GroundPrefix()
 				}
 			default:
 				continue // Wrong scope type. Bail out early.