nimdoc: anchors fix (#25601)

This fixes autogenerated references within the same-module for types,
variables and constants for custom output file names. Previously, the
module name was baked-in, now intra-module links omit the page name in
href.

In short, fixes symbol anchors for `-o:index.html`

Expected test results updated.

compiler/docgen.nim

@@ -540,10 +540,11 @@ proc nodeToHighlightedHtml(d: PDoc; n: PNode; result: var string;
       elif s != nil and s.kind in {skType, skVar, skLet, skConst} and
            sfExported in s.flags and s.owner != nil and
            belongsToProjectPackage(d.conf, s.owner) and d.target == outHtml:
-        let external = externalDep(d, s.owner)
-        result.addf "<a href=\"$1#$2\"><span class=\"Identifier\">$3</span></a>",
-          [changeFileExt(external, "html"), literal,
-           escLit]
+        let href = (if d.module == s.owner: ""
+            else: externalDep(d, s.owner).changeFileExt("html")
+          ) & "#" & literal
+        result.addf "<a href=\"$1\"><span class=\"Identifier\">$2</span></a>",
+            [href, escLit]
       else:
         dispA(d.conf, result, "<span class=\"Identifier\">$1</span>",
               "\\spanIdentifier{$1}", [escLit])