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/manual-idea-packages.nix

@@ -0,0 +1,12 @@
+{ callPackage }:
+{
+  spring-assistant = callPackage
+    ({ ideaBuild }: ideaBuild {
+      pname = "intellij-spring-assistant";
+      version = "0.12.0";
+      pluginId = 10229;
+      versionId = 44968;
+      sha256 = "13cglywzhb4j0qj0bs2jwaz2k8pxrxalv35wgkmgkxr635bxmwsj";
+    })
+    { };
+}