Fail build on unintended cross-platform imports

[amcclain]

A desktop app (or desktop Hoist components) should never import from the /mobile/ package, and vice-versa. Unfortunately this is easy to do accidentally w/IDE import suggestions and the like.

The build should detect this case and fail. I don't think we should ever have a bundle including imports from both /desktop/ and /mobile/, which seems like it would make this case easy to detect.

[amcclain]

Looks like we might also be able to get some coverage here from the linter:

https://github.com/benmosher/eslint-plugin-import/blob/master/docs/rules/no-restricted-paths.md

Did not review in depth, but appears that this would do a pretty good job in hoist-react itself where we can setup rules for the known top-level paths for desktop and mobile. Apps could do the same for their platform-specific app directories.

[amcclain]

Effectively handled via Hoist React approach in xh/hoist-react#3082