luaPackages.luarocks: generate luarocks as well

When installing a lua package that depends on luarocks, luarocks was not
capable of recognizing the luarocks module: https://github.com/NixOS/nixpkgs/issues/316009 .

As explained here https://github.com/luarocks/luarocks/issues/1659,
luarocks needs a manifest to recognize the package.
I first attempted to generate a manifest with `luarocks-admin
make-manifest` on our current manual luarocks derivation but this ended
up being too hackish as the `make-manifest` requires a .rockspec or
.rock to work, which we had not.

It seemed simpled to just rename the current manual luarocks derivation to luarocks_bootstrap
in order to break cyclic dependencies in buildLuarocksPackage and add a
generated luarocks derivation. Previously it was impossible to test
luarocks because of the cyclic dependency but we will now be able to
test the generated derivation.

pkgs/development/interpreters/lua-5/build-luarocks-package.nix

@@ -2,7 +2,7 @@
 { lib
 , lua
 , wrapLua
-, luarocks
+, luarocks_bootstrap
 , writeTextFile
 
 # Whether the derivation provides a lua module or not.
@@ -91,7 +91,7 @@ let
   nativeBuildInputs = [
     lua  # for lua.h
     wrapLua
-    luarocks
+    luarocks_bootstrap
   ];
 
   inherit doCheck extraConfig rockspecFilename knownRockspec externalDeps nativeCheckInputs;
@@ -99,7 +99,7 @@ let
   buildInputs = let
     # example externalDeps': [ { name = "CRYPTO"; dep = pkgs.openssl; } ]
     externalDeps' = lib.filter (dep: !lib.isDerivation dep) self.externalDeps;
-    in [ lua.pkgs.luarocks ]
+    in [ luarocks_bootstrap ]
       ++ buildInputs
       ++ lib.optionals self.doCheck ([ luarocksCheckHook ] ++ self.nativeCheckInputs)
       ++ (map (d: d.dep) externalDeps')