[server] Dynamically load Webpack SSR builds in Node

Summary:
Currently, if you try to run yarn cleaninstall && cd server && yarn dev, Node will crash because it expects app.build.cjs and landing.build.cjs to exist.

This was a little annoying before if you were just trying to work on native, but it's extra annoying now that the user has to rurn yarn dev in both web and landing.

This diff makes it so instead of immediately crashing, Node will defer loading these files until it gets a request for them. It also adds a helpful error message that explains what needs to be done to resolve the issue.

Test Plan: Tested web and landing on dev and prod

Reviewers: palys-swm, karol-bisztyga, atul

Reviewed By: palys-swm, atul

Subscribers: KatPo, Adrian

Differential Revision: https://phabricator.ashoat.com/D989