texlive.combine: document how to create custom packages with pkgs and tlDeps attributes

## Custom packages {#sec-language-texlive-custom-packages}

You may find that you need to use an external TeX package. A derivation for such package has to provide the contents of the "texmf" directory in its output and provide the appropriate `tlType` attribute (one of `"run"`, `"bin"`, `"doc"`, `"source"`). Dependencies on other TeX packages can be listed in the attribute `tlDeps`.

You may find that you need to use an external TeX package. A derivation for such package has to provide contents of the "texmf" directory in its output and provide the `tlType` attribute. Here is a (very verbose) example:

Such derivation must then be listed in the attribute `pkgs` of an attribute set passed to `texlive.combine`, for instance by passing `extraPkgs = { pkgs = [ custom_package ]; };`. Within Nixpkgs, `pkgs` should be part of the derivation itself, allowing users to call `texlive.combine { inherit (texlive) scheme-small; inherit some_tex_package; }`.

Here is a (very verbose) example where the attribute `pkgs` is attached to the derivation itself, which requires creating a fixed point. See also the packages `auctex`, `eukleides`, `mftrace` for more examples.

```nix

with import <nixpkgs> {};

let

  foiltex_run = stdenvNoCC.mkDerivation {

  foiltex = stdenvNoCC.mkDerivation (finalAttrs: {

    pname = "latex-foiltex";

    version = "2.1.4b";

    passthru.tlType = "run";

    passthru = {

      pkgs = [ finalAttrs.finalPackage ];

      tlDeps = with texlive; [ latex ];

      tlType = "run";

    };

    srcs = [

      (fetchurl {

      maintainers = with maintainers; [ veprbl ];

      platforms = platforms.all;

    };

  };

  foiltex = { pkgs = [ foiltex_run ]; };

  });

  latex_with_foiltex = texlive.combine {

    inherit (texlive) scheme-small;

    # now filter hyphenation patterns and formats

  (let

    hyphens = lib.filter (p: p.hasHyphens or false && p.tlType == "run") pkgList.splitBin.wrong;

    hyphenPNames = lib.sort (a: b: a < b) (map (p: p.pname) hyphens);

    hyphenPNames = map (p: p.pname) hyphens;

    formats = lib.filter (p: p.hasFormats or false && p.tlType == "run") pkgList.splitBin.wrong;

    formatPNames = lib.sort (a: b: a < b) (map (p: p.pname) formats);

    formatPNames = map (p: p.pname) formats;

    # sed expression that prints the lines in /start/,/end/ except for /end/

    section = start: end: "/${start}/,/${end}/{ /${start}/p; /${end}/!p; };\n";

    script =