discourse.mkDiscoursePlugin: Handle repos with gems directories

Some plugin repos already have a `gems` directory. This lets the
packager choose whether it should be kept and the nix packaged ruby
gems should be copied into it or if it should be removed in favor of
our ruby gems.

pkgs/servers/web-apps/discourse/default.nix

@@ -55,6 +55,7 @@ let
     , version ? null
     , meta ? null
     , bundlerEnvArgs ? {}
+    , preserveGemsDir ? false
     , src
     , ...
     }@args:
@@ -71,11 +72,20 @@ let
           runHook preInstall
           mkdir -p $out
           cp -r * $out/
-        '' + lib.optionalString (bundlerEnvArgs != {}) ''
-          ln -sf ${rubyEnv}/lib/ruby/gems $out/gems
-        '' + ''
+        '' + lib.optionalString (bundlerEnvArgs != {}) (
+          if preserveGemsDir then ''
+            cp -r ${rubyEnv}/lib/ruby/gems/* $out/gems/
+          ''
+          else ''
+            if [[ -e $out/gems ]]; then
+              echo "Warning: The repo contains a 'gems' directory which will be removed!"
+              echo "         If you need to preserve it, set 'preserveGemsDir = true'."
+              rm -r $out/gems
+            fi
+            ln -sf ${rubyEnv}/lib/ruby/gems $out/gems
+          '' + ''
           runHook postInstall
-        '';
+        '');
       });
 
   rake = runCommand "discourse-rake" {