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'