nix-zsh-completions: install nix.plugin.zsh and init.zsh as well

These files feature simple aliases and a nix-shell detector when using
`nix-shell --run zsh`. The package itself contains the completion scripts in
`$out/share/zsh/site-functions` (to keep it compatible with ZSH-only
setups) and the plugins in `$out/share/zsh/plugins` for oh-my-zsh.

pkgs/shells/zsh/nix-zsh-completions/default.nix

@@ -15,15 +15,16 @@ stdenv.mkDerivation rec {
   };
 
   installPhase = ''
-    mkdir -p $out/share/zsh/site-functions
+    mkdir -p $out/share/zsh/{site-functions,plugins/nix}
     cp _* $out/share/zsh/site-functions
+    cp *.zsh $out/share/zsh/plugins/nix
   '';
 
-  meta = {
+  meta = with stdenv.lib; {
     homepage = https://github.com/spwhitt/nix-zsh-completions;
     description = "ZSH completions for Nix, NixOS, and NixOps";
-    license = stdenv.lib.licenses.bsd3;
-    platforms = stdenv.lib.platforms.all;
-    maintainers = [ stdenv.lib.maintainers.spwhitt stdenv.lib.maintainers.olejorgenb stdenv.lib.maintainers.hedning ];
+    license = licenses.bsd3;
+    platforms = platforms.all;
+    maintainers = with maintainers; [ spwhitt olejorgenb hedning ma27 ];
   };
 }