Don't use app node_modules folder as a resolve fallback

[gaearon]

This behavior was originally introduced in #1359 as we thought it would fix the npm link development workflow issues.

However, we have since found several problems with it:

• It doesn’t match the Node resolution mechanism (which reverted a similar change in module: add --preserve-symlinks command line flag nodejs/node#6537 and kept it behind a flag)
• It still leads to the “duplicate React” problem (Duplicate React #675)
• It is inconsistent with Jest (as described in Resolve symlinks using Node resolution mechanism #3883, past versions of Jest had this behavior but it led to issues like Jest requires symlinked files twice jestjs/jest#3830 and Preserve module identity for symlinks jestjs/jest#4761 (comment) so I fixed it in Preserve module identity for symlinks jestjs/jest#4761 to match the Node resolution mechanism)

There are also gotchas like webpack/webpack#985 (comment), and I feel like it’s only going to get worse once we start supporting monorepos (#3741).

I propose that we completely disable this behavior for now. People who really need it can use an escape hatch: add NODE_PATH=node_modules to their .env file. This restores the 1.x behavior (and conveniently works in Jest too).

Our recommendation in any case will be to use monorepos instead (#3741) and stop relying on npm link.

[gaearon]

cc @bradfordlemley

[Timer]

As a separate PR, I think we should normalize the path delimiter for macOS & Windows support.

Many people commit their .env files and I see this becoming commonplace:
NODE_PATH=src;node_modules