Cli: functions: don't allow requiring shared/external folders

[sw-yx]

- Do you want to request a feature or report a bug?
bug

- What is the current behavior?

netlify dev allows functions requiring external modules because it is just node. but netlify prod doesn't. so we should disable this or warn very loudly.

https://community.netlify.com/t/requiring-modules-external-to-functions-directory/1231/2

[fool]

affected customers to inform upon fix:

https://community.netlify.com/t/requiring-modules-external-to-functions-directory/1231/2

[sw-yx]

bump cc @raeesbhatti

here's a repro! https://github.com/sw-yx/repro-shared-functions-bug

[revett]

+1 would be great to see this functionality, functions end up duplicated a lot of code if not.

[sw-yx]

feel free to tackle it if you have an idea how to do it @revett this has not been prioritzed at all

im thinking maybe mess with node module resolution in netlify dev on a per function basis