Shared: Improve makeSections.

Previously it would sometimes create doubled section divs.
By leaving the inner heading's id on the heading itself, and
then consolidating the inner and outer section divs, we can
avoid this undesirable result.

Closes #11745.

test/command/11745.md

@@ -0,0 +1,26 @@
+```
+% pandoc --section-divs
+# Book Title
+
+::: {#ch1 .section}
+## Chapter 1
+Hello world.
+:::
+
+::: {#ch2 .section}
+## Chapter 2
+Goodbye world.
+:::
+^D
+<section id="book-title" class="level1">
+<h1>Book Title</h1>
+<section id="ch1" class="level2">
+<h2 id="chapter-1">Chapter 1</h2>
+<p>Hello world.</p>
+</section>
+<section id="ch2" class="level2">
+<h2 id="chapter-2">Chapter 2</h2>
+<p>Goodbye world.</p>
+</section>
+</section>
+```