* Move Code to a submodule
Closes#4901.
* Base Code cache on hash and re-enable node_modules cache
The current setup appears to only rebuild VS Code if the dependencies
change but we need to rebuild it if anything changes.
I also re-enabled the commented out node_modules caches. They look like
they should work to me with the submodule method. I think the problem
occurred because Code itself was being installed in the yarn step.
