docgen: implement cross-document links (#20990)

* docgen: implement cross-document links

Fully implements https://github.com/nim-lang/RFCs/issues/125
Follow-up of: https://github.com/nim-lang/Nim/pull/18642 (for internal links)
and https://github.com/nim-lang/Nim/issues/20127.

Overview
--------

Explicit import-like directive is required, called `.. importdoc::`.
(the syntax is % RST, Markdown will use it for a while).

Then one can reference any symbols/headings/anchors, as if they
were in the local file (but they will be prefixed with a module name
or markup document in link text).
It's possible to reference anything from anywhere (any direction
in `.nim`/`.md`/`.rst` files).

See `doc/docgen.md` for full description.

Working is based on `.idx` files, hence one needs to generate
all `.idx` beforehand. A dedicated option `--index:only` is introduced
(and a separate stage for `--index:only` is added to `kochdocs.nim`).

Performance note
----------------

Full run for `./koch docs` now takes 185% of the time before this PR.
(After: 315 s, before: 170 s on my PC).
All the time seems to be spent on `--index:only` run, which takes
almost as much (85%) of normal doc run -- it seems that most time
is spent on file parsing, turning off HTML generation phase has not
helped much.
(One could avoid it by specifying list of files that can be referenced
and pre-processing only them. But it can become error-prone and I assume
that these linke will be **everywhere** in the repository anyway,
especially considering https://github.com/nim-lang/RFCs/issues/478.
So every `.nim`/`.md` file is processed for `.idx` first).

But that's all without significant part of repository converted to
cross-module auto links. To estimate impact I checked the time for
`doc`ing a few files (after all indexes have been generated), and
everywhere difference was **negligible**.
E.g. for `lib/std/private/osfiles.nim` that `importdoc`s large
`os.idx` and hence should have been a case with relatively large
performance impact, but:

* After: 0.59 s.
* Before: 0.59 s.

So Nim compiler works so slow that doc part basically does not matter :-)

Testing
-------

1) added `extlinks` test to `nimdoc/`
2) checked that `theindex.html` is still correct
2) fixed broken auto-links for modules that were derived from `os.nim`
   by adding appropriate ``importdoc``

Implementation note
-------------------

Parsing and formating of `.idx` entries is moved into a dedicated
`rstidx.nim` module from `rstgen.nim`.

`.idx` file format changed:

* fields are not escaped in most cases because we need original
  strings for referencing, not HTML ones
  (the exception is linkTitle for titles and headings).
  Escaping happens later -- on the stage of `rstgen` buildIndex, etc.
* all lines have fixed number of columns 6
* added discriminator tag as a first column,
  it always allows distinguish Nim/markup entries, titles/headings, etc.
  `rstgen` does not rely any more (in most cases) on ad-hoc logic
  to determine what type each entry is.
* there is now always a title entry added at the first line.
* add a line number as 6th column
* linkTitle (4th) column has a different format: before it was like
  `module: funcName()`, now it's `proc funcName()`.
  (This format is also propagated to `theindex.html` and search results,
  I kept it that way since I like it more though it's discussible.)
  This column is what used for Nim symbols resolution.
* also changed details on column format for headings and titles:
  "keyword" is original, "linkTitle" is HTML one

* fix paths on Windows + more clear code

* Update compiler/docgen.nim

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

* Handle .md and .nim paths uniformly in findRefFile

* handle titles better + more comments

* don't allow markup overwrite index title for .nim files

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

lib/std/appdirs.nim

@@ -1,5 +1,7 @@
 ## This module implements helpers for determining special directories used by apps.
 
+## .. importdoc:: paths.nim
+
 from std/private/osappdirs import nil
 import std/paths
 import std/envvars

lib/std/private/osappdirs.nim

@@ -1,3 +1,5 @@
+## .. importdoc:: paths.nim, dirs.nim
+
 include system/inclrtl
 import std/envvars
 import std/private/ospaths2

lib/std/private/oscommon.nim

@@ -5,6 +5,8 @@ import std/[oserrors]
 when defined(nimPreviewSlimSystem):
   import std/[syncio, assertions, widestrs]
 
+## .. importdoc:: osdirs.nim, os.nim
+
 const weirdTarget* = defined(nimscript) or defined(js)
 
 

lib/std/private/osdirs.nim

@@ -1,3 +1,5 @@
+## .. importdoc:: osfiles.nim, appdirs.nim, paths.nim
+
 include system/inclrtl
 import std/oserrors
 

lib/std/private/osfiles.nim

@@ -7,6 +7,7 @@ export fileExists
 
 import ospaths2, ossymlinks
 
+## .. importdoc:: osdirs.nim, os.nim
 
 when defined(nimPreviewSlimSystem):
   import std/[syncio, assertions, widestrs]
@@ -420,4 +421,4 @@ proc moveFile*(source, dest: string) {.rtl, extern: "nos$1",
         removeFile(source)
       except:
         discard tryRemoveFile(dest)
-        raise
+        raise

lib/std/private/ospaths2.nim

@@ -10,6 +10,8 @@ export ReadDirEffect, WriteDirEffect
 when defined(nimPreviewSlimSystem):
   import std/[syncio, assertions, widestrs]
 
+## .. importdoc:: osappdirs.nim, osdirs.nim, osseps.nim, os.nim
+
 const weirdTarget = defined(nimscript) or defined(js)
 
 when weirdTarget:

lib/std/private/osseps.nim

@@ -3,6 +3,8 @@
 
 # Improved based on info in 'compiler/platform.nim'
 
+## .. importdoc:: ospaths2.nim
+
 const
   doslikeFileSystem* = defined(windows) or defined(OS2) or defined(DOS)
 

lib/std/private/ossymlinks.nim

@@ -31,6 +31,7 @@ elif defined(js):
 else:
   {.pragma: noNimJs.}
 
+## .. importdoc:: os.nim
 
 proc createSymlink*(src, dest: string) {.noWeirdTarget.} =
   ## Create a symbolic link at `dest` which points to the item specified

lib/std/symlinks.nim

@@ -1,10 +1,11 @@
 ## This module implements symlink (symbolic link) handling.
 
+## .. importdoc:: os.nim
+
 from paths import Path, ReadDirEffect
 
 from std/private/ossymlinks import symlinkExists, createSymlink, expandSymlink
 
-
 proc symlinkExists*(link: Path): bool {.inline, tags: [ReadDirEffect].} =
   ## Returns true if the symlink `link` exists. Will return true
   ## regardless of whether the link points to a directory or file.