jetbrains: rework following the Emacs builder

This commit moves a lot of the heavy lifting out of callPackage and back
into regular import, following conventions from upstream Nixpkgs. It
allows for a clearer and less magic definition of new packages,
particularly by downstream, as well, by exposing the *Build package
callers in the attribute set.

pkgs/applications/editors/jetbrains/wrapper.nix

@@ -0,0 +1,30 @@
+{ lib
+}: self:
+
+with lib;
+
+package:
+pluginsFun:
+
+let
+  plugins =
+    if isFunction pluginsFun
+    then pluginsFun self
+    else pluginsFun;
+
+  # FIXME: Is this still needed?
+  info = builtins.parseDrvName package.name;
+  badPlugins = filter (p: ! elem info.name p.jetbrainsPlatforms) plugins;
+  errorMsg = "plugins [ ${toString (map (p: p.name) badPlugins)} ] are not available for platform ${info.name}";
+in
+
+assert assertMsg (length badPlugins == 0) errorMsg;
+
+appendToName "with-plugins" (package.overrideAttrs (oldAttrs: {
+  inherit plugins;
+  installPhase = oldAttrs.installPhase + ''
+    for plugin in $plugins; do
+      ln -s "$plugin" "$out/$name/plugins/$(basename $plugin)"
+    done
+  '';
+}))