RelaxNG XML-syntax parser does not traverse <div> when collecting <define>s

[casoon]

Summary

parse_relaxng (src/validation/relaxng.rs) builds its define/symbol table
by scanning the direct children of <grammar>, but does not recurse into
<div> wrapper elements. Per the RELAX NG spec, <div> is purely
organizational — <define>s nested inside a <div> belong to the
enclosing <grammar>'s single flat symbol table, exactly like a top-level
<define>.

As a result, any <define> nested inside one or more <div> levels is
invisible to name resolution, and a <ref name="..."> pointing to it fails
with undefined reference: ... even though the definition is present in
the document.

This shows up in practice when flattening a modular RELAX NG schema (e.g.
via rnginline, which resolves <include>/<externalRef> by wrapping
each resolved module's content in a <div>) into a single file for
xmloxide to consume — the flattener's output is spec-valid RNG, but
xmloxide can't resolve refs into it until the <div> wrappers are
manually stripped/unwrapped first.

Reproduction

use xmloxide::validation::relaxng::parse_relaxng;

let schema = parse_relaxng(r#"
  <grammar xmlns="http://relaxng.org/ns/structure/1.0">
    <start><ref name="root"/></start>
    <div>
      <define name="root"><element name="root"><empty/></element></define>
    </div>
  </grammar>
"#).unwrap();

Expected: schema parses and can validate <root/>.
Actual: fails (or, if parse_relaxng itself succeeds, validate errors
with undefined reference: root) because the <define> inside <div>
was never collected.

Suggested fix

When collecting <define> children of <grammar> (and recursively for
nested <div>s), treat <div> as transparent: recurse into it and
collect its <define>/<start> children into the same enclosing scope,
per RELAX NG §4.16 ("div").

Context

Found while vendoring the W3C/WHATWG HTML5 RELAX NG schema from
validator/validator (the Nu Html Checker) for
https://github.com/casoon/html-conform — that schema is naturally modular
(19 included sub-grammars) and any general-purpose flattening tool
produces <div>-wrapped output.

xmloxide version: 0.5.0