message: refactor: distinguish between in and out packages
diff: const {merge, pick} = require("lodash");
  -const SLOTS = ["head", "css", "before", "html", "after", "js"];
  +const IN_SLOTS = ["html", "css", "js", "md", "json"];
  +const OUT_SLOTS = [...IN_SLOTS, "head", "before", "after", "script"];
  module.exports = render;
  -module.exports.SLOTS = SLOTS;
  function render(input) {
  - return SLOTS
  + return IN_SLOTS
  .reduce((output, slot) => {
  execute(input, output, {name: slot});
  return output;
  @@ -28,7 +28,7 @@ function execute(input, output, {name}) {
  }
  if (typeof result === "object" && !Array.isArray(result)) {
  - merge(output, pick(result, SLOTS));
  + merge(output, pick(result, OUT_SLOTS));
  return output;
  }
  