Revert "HHH-14096 Remove unused import for deprecated type XmlDocument and dom4j"

Imports are required for 5.3 branch

This reverts commit 5ead0e4dd5fb0ca1a2902718de660718b88ffa8f.

hibernate-core/src/main/java/org/hibernate/InvalidMappingException.java

@@ -7,6 +7,7 @@
 package org.hibernate;
 
 import org.hibernate.boot.jaxb.Origin;
+import org.hibernate.internal.util.xml.XmlDocument;
 
 /**
  * Thrown when a mapping is found to be invalid.

hibernate-core/src/main/java/org/hibernate/internal/util/xml/XmlDocument.java

@@ -8,6 +8,8 @@
 
 import java.io.Serializable;
 
+import org.dom4j.Document;
+
 /**
  * Describes a parsed xml document.
  *