schema REFACTOR rename lys_find_ext_instance_node() to lysc_ext_find_node()

Also add description of the function and move the declaration to the more
appropriate place in source codes.
diff --git a/src/parser_xml.c b/src/parser_xml.c
index 33235a0..8bed12f 100644
--- a/src/parser_xml.c
+++ b/src/parser_xml.c
@@ -438,7 +438,7 @@
     snode = NULL;
     if (mod && (!parent || parent->schema)) {
         if (!parent && lydctx->ext) {
-            snode = lys_find_ext_instance_node(lydctx->ext, mod, name, name_len, 0, getnext_opts);
+            snode = lysc_ext_find_node(lydctx->ext, mod, name, name_len, 0, getnext_opts);
         } else {
             snode = lys_find_child(parent ? parent->schema : NULL, mod, name, name_len, 0, getnext_opts);
         }