treewide: rename openmpt123 to libopenmpt to match the name in other distros

pkgs/applications/audio/audacious/default.nix

···
   libcddb, libcdio, libcdio-paranoia, libcue, libjack2, libmad, libmms, libmodplug,
   libmowgli, libnotify, libogg, libpulseaudio, libsamplerate, libsidplayfp,
   libsndfile, libvorbis, libxml2, lirc, mpg123, neon, qtmultimedia, soxr,
-  wavpack, openmpt123
+  wavpack, libopenmpt
 }:
 
 mkDerivation rec {
···
     libcdio libcdio-paranoia libcue libjack2 libmad libmms libmodplug libmowgli
     libnotify libogg libpulseaudio libsamplerate libsidplayfp libsndfile
     libvorbis libxml2 lirc mpg123 neon qtmultimedia soxr wavpack
-    openmpt123
+    libopenmpt
   ];
 
   # Here we build both audacious and audacious-plugins in one

pkgs/applications/audio/openmpt123/default.nix pkgs/applications/audio/libopenmpt/default.nix

···
 , usePulseAudio ? config.pulseaudio or false, libpulseaudio }:
 
 stdenv.mkDerivation rec {
-  pname = "openmpt123";
+  pname = "libopenmpt";
   version = "0.5.10";
 
   src = fetchurl {

pkgs/top-level/aliases.nix

···
   openisns = open-isns; # added 2020-01-28
   openjpeg_1 = throw "openjpeg_1 has been removed, use openjpeg_2 instead"; # added 2021-01-24
   openjpeg_2 = openjpeg; # added 2021-01-25
+  openmpt123 = libopenmpt; # added 2021-09-05
   opensans-ttf = open-sans; # added 2018-12-04
   openssh_with_kerberos = openssh; # added 2018-01-28
   orchis = orchis-theme; # added 2021-06-09

pkgs/top-level/all-packages.nix

···
 
   vivaldi-widevine = callPackage ../applications/networking/browsers/vivaldi/widevine.nix { };
 
-  openmpt123 = callPackage ../applications/audio/openmpt123 { };
+  libopenmpt = callPackage ../applications/audio/libopenmpt { };
 
   openrazer-daemon = with python3Packages; toPythonApplication openrazer-daemon;