entc/gen: support merging dep annotations

This will make it possible to get dependencies
from external extensions.

dialect/entsql/annotation.go

@@ -159,10 +159,10 @@ func (a Annotation) Merge(other schema.Annotation) schema.Annotation {
 	return a
 }
 
-var (
-	_ schema.Annotation = (*Annotation)(nil)
-	_ schema.Merger     = (*Annotation)(nil)
-)
+var _ interface {
+	schema.Annotation
+	schema.Merger
+} = (*Annotation)(nil)
 
 // ReferenceOption for constraint actions.
 type ReferenceOption string