schema parsing CHANGE try to differentiate XPath errors

We are checking XPath expressions (of must and when) already when parsing
the schema. If the mentioned nodes are not found in the schemas, the
expression will always fail. Now we try to differentiate situation, when
it is certain error (the expression refers the node in the same schema
which is missing) and when it is possibly caused by not implementing another
module (the expresison refers the node from another module and this node is
not present). In the second case, we not report the issue as error, but only
as a warning. The schema is then correctly parsed, but the data cannot
appear anyway, since the must/when expression will always fail.
diff --git a/src/tree_schema.c b/src/tree_schema.c
index 7484a99..cc7cacc 100644
--- a/src/tree_schema.c
+++ b/src/tree_schema.c
@@ -2634,12 +2634,82 @@
     return NULL;
 }
 
+int
+lys_has_xpath(const struct lys_node *node)
+{
+    assert(node);
+
+    switch (node->nodetype) {
+    case LYS_AUGMENT:
+        if (((struct lys_node_augment *)node)->when) {
+            return 1;
+        }
+        break;
+    case LYS_CASE:
+        if (((struct lys_node_case *)node)->when) {
+            return 1;
+        }
+        break;
+    case LYS_CHOICE:
+        if (((struct lys_node_choice *)node)->when) {
+            return 1;
+        }
+        break;
+    case LYS_ANYDATA:
+        if (((struct lys_node_anydata *)node)->when || ((struct lys_node_anydata *)node)->must_size) {
+            return 1;
+        }
+        break;
+    case LYS_LEAF:
+        if (((struct lys_node_leaf *)node)->when || ((struct lys_node_leaf *)node)->must_size) {
+            return 1;
+        }
+        break;
+    case LYS_LEAFLIST:
+        if (((struct lys_node_leaflist *)node)->when || ((struct lys_node_leaflist *)node)->must_size) {
+            return 1;
+        }
+        break;
+    case LYS_LIST:
+        if (((struct lys_node_list *)node)->when || ((struct lys_node_list *)node)->must_size) {
+            return 1;
+        }
+        break;
+    case LYS_CONTAINER:
+        if (((struct lys_node_container *)node)->when || ((struct lys_node_container *)node)->must_size) {
+            return 1;
+        }
+        break;
+    case LYS_INPUT:
+    case LYS_OUTPUT:
+        if (((struct lys_node_inout *)node)->must_size) {
+            return 1;
+        }
+        break;
+    case LYS_NOTIF:
+        if (((struct lys_node_notif *)node)->must_size) {
+            return 1;
+        }
+        break;
+    case LYS_USES:
+        if (((struct lys_node_uses *)node)->when) {
+            return 1;
+        }
+        break;
+    default:
+        /* does not have XPath */
+        break;
+    }
+
+    return 0;
+}
+
 struct lys_node *
 lys_node_dup(struct lys_module *module, struct lys_node *parent, const struct lys_node *node, uint8_t nacm,
              struct unres_schema *unres, int shallow)
 {
     struct lys_node *p = NULL;
-    int finalize = 0, has_xpath;
+    int finalize = 0;
     struct lys_node *result, *iter, *next;
 
     if (!shallow) {
@@ -2654,69 +2724,7 @@
     if (finalize) {
         /* check xpath expressions in the instantiated tree */
         for (iter = next = parent->child; iter; iter = next) {
-            has_xpath = 0;
-            switch (iter->nodetype) {
-            case LYS_AUGMENT:
-                if (((struct lys_node_augment *)iter)->when) {
-                    has_xpath = 1;
-                }
-                break;
-            case LYS_CASE:
-                if (((struct lys_node_case *)iter)->when) {
-                    has_xpath = 1;
-                }
-                break;
-            case LYS_CHOICE:
-                if (((struct lys_node_choice *)iter)->when) {
-                    has_xpath = 1;
-                }
-                break;
-            case LYS_ANYDATA:
-                if (((struct lys_node_anydata *)iter)->when || ((struct lys_node_anydata *)iter)->must_size) {
-                    has_xpath = 1;
-                }
-                break;
-            case LYS_LEAF:
-                if (((struct lys_node_leaf *)iter)->when || ((struct lys_node_leaf *)iter)->must_size) {
-                    has_xpath = 1;
-                }
-                break;
-            case LYS_LEAFLIST:
-                if (((struct lys_node_leaflist *)iter)->when || ((struct lys_node_leaflist *)iter)->must_size) {
-                    has_xpath = 1;
-                }
-                break;
-            case LYS_LIST:
-                if (((struct lys_node_list *)iter)->when || ((struct lys_node_list *)iter)->must_size) {
-                    has_xpath = 1;
-                }
-                break;
-            case LYS_CONTAINER:
-                if (((struct lys_node_container *)iter)->when || ((struct lys_node_container *)iter)->must_size) {
-                    has_xpath = 1;
-                }
-                break;
-            case LYS_INPUT:
-            case LYS_OUTPUT:
-                if (((struct lys_node_inout *)iter)->must_size) {
-                    has_xpath = 1;
-                }
-                break;
-            case LYS_NOTIF:
-                if (((struct lys_node_notif *)iter)->must_size) {
-                    has_xpath = 1;
-                }
-                break;
-            case LYS_USES:
-                if (((struct lys_node_uses *)iter)->when) {
-                    has_xpath = 1;
-                }
-                break;
-            default:
-                /* do nothing, has_xpath is zero */;
-                break;
-            }
-            if (has_xpath && unres_schema_add_node(module, unres, iter, UNRES_XPATH, NULL) == -1) {
+            if (lys_has_xpath(iter) && unres_schema_add_node(module, unres, iter, UNRES_XPATH, NULL) == -1) {
                 /* invalid xpath */
                 return NULL;
             }