Don't rely on global/$PATH react-native-cli

[clayallsopp]

I (and others) dislike installing dependencies globally. My workflow with react-native is to add the cli to the package.json so that the versions are locked for each project, etc - general reasons why global dependencies are avoided

The current Xcode build script expects react-native to be on the Xcode shell's $PATH, which can be troublesome (lots of issues related to this, i.e. #3948). This PR adds support for two things:

1. Automatically detects react-native-cli installed via package.json instead of globally
2. Allows a REACT_NATIVE_PATH variable that can be set via Xcode if your package exists elsewhere (not sure how helpful this would be for other folks, but hey)

Happy to explore other implementations, but the gist here is that generated react-native projects should be able to work with a project-specific version of react-native-cli

[facebook-github-bot]

By analyzing the blame information on this pull request, we identified @frantic, @ide and @lpil to be potential reviewers.

[lpil]

I like this change a lot. :)

[brentvatne]

cc @martinbigio

[ide]

the reliance on cwd here might be fragile. just a thought.

[clayallsopp]

The generated Xcode project's build step also relies on cwd, so (I think) it's a consistent assumption:

[frantic]

We rely on cwd inside Xcode because I don't see any other way. In react-native-xcode.sh we can actually resolve the path to local-cli/cli.js relative to packager/react-native-xcode.sh location.

[martinbigio]

@frantic could you take a look?

[frantic]

Hey @clayallsopp, thanks for the pull request! You mentioned that you'd be happy to explore other suggestions, and I think I have one for you that would solve a lot of problems with react-native-xcode.sh.

react-native-cli is a very tiny package that does nothing but delegates all commands to locally installed react-native/cli.js, which ends up calling react-native/local-cli/cli.js.

Instead of relying on globally installed react-native command, we can just invoke local-cli/cli.js directly.

[frantic]

FYI: this is a dummy binary that warns users that they have installed react-native globally (instead of react-native-cli)

[frantic]

Closing in favor of #5518, @clayallsopp thanks again for bringing up this topic!