yt-dlp: add option to install a youtube-dl alias

youtube-dl development seems to have stalled and yt-dlp is the de-facto upstream
nowadays. This provides an easy way to use yt-dlp as a drop-in replacement:

    youtube-dl = yt-dlp.override { withAlias = true; };

pkgs/tools/misc/yt-dlp/default.nix

@@ -12,6 +12,7 @@
 , rtmpSupport ? true
 , phantomjsSupport ? false
 , hlsEncryptedSupport ? true
+, withAlias ? false # Provides bin/youtube-dl for backcompat
 }:
 
 buildPythonPackage rec {
@@ -50,6 +51,10 @@ buildPythonPackage rec {
   # Requires network
   doCheck = false;
 
+  postInstall = lib.optionalString withAlias ''
+      ln -s "$out/bin/yt-dlp" "$out/bin/youtube-dl"
+  '';
+
   meta = with lib; {
     homepage = "https://github.com/yt-dlp/yt-dlp/";
     description = "Command-line tool to download videos from YouTube.com and other sites (youtube-dl fork)";