Markdown reader: avoid duplicate ids with auto_identifiers.

We previously avoided generating a duplicate with another automatically
generated identifier; now we also avoid duplicates with explicit
identifiers that occur before the header for which an identifier
is being generated.  (Collisions are still possible for identifiers
that occur after the header.)

T.P.Shared: `makeSections` is also modified so it doesn't give
bad results when the enclosing Div has a different identifier
from the header, as may now happen.

test/command/6384.md

@@ -1,16 +1,16 @@
 ```
 % pandoc --wrap=preserve --file-scope command/file1.txt command/file2.txt
 ^D
-<div id="command__file1.txt">
+<section id="command__file1.txt">
 <h1 id="command__file1.txt__zed">Zed</h1>
 <p><a href="bar">foo</a>
 and <a href="#command__file1.txt__zed">Zed</a>
 and <a href="#command__file2.txt__zed">other Zed</a>
 and <a href="#command__file2.txt">other file</a>
 and <a href="c.md#zed">foreign Zed</a></p>
-</div>
-<div id="command__file2.txt">
+</section>
+<section id="command__file2.txt">
 <h2 id="command__file2.txt__zed">Zed</h2>
 <p><a href="baz">foo</a></p>
-</div>
+</section>
 ```

test/command/section-divs.md

@@ -14,12 +14,12 @@ Ok
 :::
 ^D
 <section id="hi" class="level1">
-<h1>Hi</h1>
+<h1 id="hi-1">Hi</h1>
 <section id="there" class="level2">
-<h2>there</h2>
+<h2 id="there-1">there</h2>
 </section>
 </section>
 <section id="ok" class="level1">
-<h1>Ok</h1>
+<h1 id="ok-1">Ok</h1>
 </section>
 ```