parsers: re-arrange data_forms validation implementation

This is more in line with how we handle closely coupled specifications
already. While there are subdirectories for "large" specifications (such
as MUC and PubSub), those only refer to a single XEP document. When
there are multiple separate XEP documents, we have separate modules for
that.

parsers/src/lib.rs

@@ -120,6 +120,9 @@ pub mod caps;
 /// XEP-0118: User Tune
 pub mod tune;
 
+/// XEP-0122: Data Forms Validation
+pub mod data_forms_validate;
+
 ///XEP-0153: vCard-Based Avatars
 pub mod vcard_update;