yarn2nix: remove more mentions of it (#493108)

##### mixRelease - Frontend dependencies (javascript) {#mix-release-javascript-deps}

For phoenix projects, inside of Nixpkgs you can either use yarn2nix (mkYarnModule) or node2nix. An example with yarn2nix can be found [here](https://github.com/NixOS/nixpkgs/blob/master/pkgs/servers/web-apps/plausible/default.nix#L39). An example with node2nix will follow. To package something outside of nixpkgs, you have alternatives like [npmlock2nix](https://github.com/nix-community/npmlock2nix) or [nix-npm-buildpackage](https://github.com/serokell/nix-npm-buildpackage)

For phoenix projects, inside of Nixpkgs you can either use `fetchYarnDeps` or `buildNpmPackage`. An example with `fetchYarnDeps` can be found [here](https://github.com/NixOS/nixpkgs/blob/master/pkgs/by-name/pl/plausible/package.nix). An example with `fetchYarnDeps` will follow. To package something outside of nixpkgs, you have alternatives like [npmlock2nix](https://github.com/nix-community/npmlock2nix) or [nix-npm-buildpackage](https://github.com/serokell/nix-npm-buildpackage)

##### mixRelease - backend dependencies (mix) {#mix-release-mix-deps}

Each tool has an abstraction to just build the node_modules (dependencies) directory.

You can always use the `stdenv.mkDerivation` with the node_modules to build the package (symlink the node_modules directory and then use the package build command).

The node_modules abstraction can be also used to build some web framework frontends.

For an example of this see how [plausible](https://github.com/NixOS/nixpkgs/blob/master/pkgs/by-name/pl/plausible/package.nix) is built.

Then when building the frontend you can just symlink the node_modules directory.

## Tool-specific instructions {#javascript-tool-specific}

const path = require('path')

// This has to match the logic in pkgs/development/tools/yarn2nix-moretea/yarn2nix/lib/urlToName.js

// so that fixup_yarn_lock produces the same paths

const urlToName = url => {

  const isCodeloadGitTarballUrl = url.startsWith('https://codeload.github.com/') && url.includes('/tar.gz/')

  copyDesktopItems,

}:

# NOTE mqtt-explorer has 3 yarn subpackages and uses relative links

# between them, which makes it hard to package them via 3 `mkYarnPackage`

# between them, which makes it hard to package them via 3 `fetchYarnDeps`

# since the resulting `node_modules` directories don't have the same structure

# as if they were installed directly. Hence why we opted to use a

# `stdenv.mkDerivation` instead.

    # of better-sqlite3. It has a native part that it wants to build using a

    # script which is disallowed.

    # What's more, we need to use headers from electron to avoid ABI mismatches.

    # Adapted from mkYarnModules.

    for f in $(find . -path '*/node_modules/better-sqlite3' -type d); do

      (cd "$f" && (

      npm run build-release --offline --nodedir="${electron.headers}"