plugins/lsp: Extract rust-analyzer options from the source directly (#562)

This directly reads the package.json file in pkgs.rust-analyzer.src in
order to generate the list of options for rust-analyzer.

This avoids the need for a generator script, and makes sure that it is
always in sync.

helpers/default.nix

@@ -1,5 +1,4 @@
 pkgs: rec {
-  rust-analyzer-config = pkgs.callPackage ./rust-analyzer {};
   efmls-configs-tools = pkgs.callPackage ./efmls-configs {};
   autogenerated-configs = pkgs.callPackage ({stdenv}:
     stdenv.mkDerivation {
@@ -11,7 +10,6 @@ pkgs: rec {
 
       installPhase = ''
         mkdir -p $out
-        cp ${rust-analyzer-config}/share/* $out
         cp ${efmls-configs-tools}/share/* $out
       '';
     }) {};