create separate bundles for external modules

• With the support of bundling external packages, we bundle the included packages along with the source journey files to be able to run them via Heartbeat later without installing any packages. This is good, but it changes the source line numbers and column numbers of those particular journeys and might be a bad UX if we have to go back to the original source.

More details in here - https://github.com/elastic/synthetics/pull/580#pullrequestreview-1082955330

The ultimate goal of our push command is to replicate exactly how your folder structure is on your local and run the same inside HB when pushed. This would make it presentable in Kibana when we add unzipping support and allow users to view and edit monitors.

TLDR: If i push below example journey code

// examples/bundled-code/test.journey.ts
import { journey, step, monitor, expect } from "@elastic/synthetics";
import isPositive from "is-positive";
import leftpad from "leftpad";
import * as utils from "./utils";

journey("example journey", ({ page, params }) => {
  step("example step", () => {});
});

Our bundled zip folder should be in the below format.

examples/bundled-code/test.journey.ts
examples/bundled-code/utils.ts
examples/bundled-code/is-positive.js
examples/bundled-code/leftpad.js

Start by tracing the push command's bundling flow and read the linked pull request review for the existing external-package behavior. Use the example in the issue as the acceptance case; done means the pushed zip preserves the journey and local utils while placing external modules such as is-positive.js and leftpad.js alongside them, without losing source structure.