Fix auto links to subheader when TOC is present (#20279)

Fix links to subheader when TOC is present

It was observed (in https://github.com/nim-lang/Nim/pull/20112)
that links to 2nd- (and subsequent) -level headings
fail if TOC is present, e.g.:
```nim
.. contents::

Type relations
==============

Convertible relation
--------------------

Ref. `Convertible relation`_
```

The problem here is that links are resolved in `rst.nim` but later
`rstgen.nim` fixes ("fixes") anchors to make them unique so that
TOC always works (if e.g. there was another sub-section like
"Convertible relation").
The solution implemented in this PR is to move that fix-up of anchors
into `rst.nim`, so that link resolution could know final anchors.

The bug seems to be added in https://github.com/nim-lang/Nim/pull/2332
in 2015, that is it is present in Nim 1.0.

tests/stdlib/trst.nim

@@ -74,7 +74,7 @@ suite "RST parsing":
         """.toAst ==
       dedent"""
         rnInner
-          rnHeadline  level=1
+          rnHeadline  level=1  anchor='lexical-analysis'
             rnLeaf  'Lexical'
             rnLeaf  ' '
             rnLeaf  'Analysis'

tests/stdlib/trstgen.nim

@@ -536,6 +536,63 @@ Some chapter
     let output1 = input1.toHtml
     doAssert output1 == "GC_step"
 
+  test "RST anchors/links to headings":
+    # Currently in TOC mode anchors are modified (for making links from
+    # the TOC unique)
+    let inputNoToc = dedent"""
+        Type relations
+        ==============
+
+        Convertible relation
+        --------------------
+
+        Ref. `Convertible relation`_
+        """
+    let outputNoToc = inputNoToc.toHtml
+    check outputNoToc.count("id=\"type-relations\"") == 1
+    check outputNoToc.count("id=\"convertible-relation\"") == 1
+    check outputNoToc.count("href=\"#convertible-relation\"") == 1
+
+    let inputTocCases = @[
+      dedent"""
+        .. contents::
+
+        Type relations
+        ==============
+
+        Convertible relation
+        --------------------
+
+        Ref. `Convertible relation`_
+
+        Guards and locks
+        ================
+        """,
+      dedent"""
+        Ref. `Convertible relation`_
+
+        .. contents::
+
+        Type relations
+        ==============
+
+        Convertible relation
+        --------------------
+
+        Guards and locks
+        ================
+        """
+    ]
+    for inputToc in inputTocCases:
+      let outputToc = inputToc.toHtml
+      check outputToc.count("id=\"type-relations\"") == 1
+      check outputToc.count("id=\"type-relations-convertible-relation\"") == 1
+      check outputToc.count("id=\"convertible-relation\">") == 0
+      # Besides "Ref.", heading also contains link to itself:
+      check outputToc.count(
+          "href=\"#type-relations-convertible-relation\">") == 2
+      check outputToc.count("href=\"#convertible-relation\"") == 0
+
   test "RST links":
     let input1 = """
 Want to learn about `my favorite programming language`_?