[cli][upgrade] Use Yarn if available

[mkonicek]

Motivation

If the project is using yarn to manage its dependencies, we should be running 'yarn add' to upgrade the React Native dependency, rather than 'npm install'.

Test plan (required)

Running in a project that's in a bad state:

Error: react-native version in "package.json" (0.29.2) doesn't match the installed version in "node_modules" (0.38.0).
Try running "yarn" to fix this.

Removed yarn.lock file, ran again:

Error: react-native version in "package.json" (0.29.2) doesn't match the installed version in "node_modules" (0.38.0).
Try running "npm install" to fix this.

Running inside a folder that doesn't contain a package.json file:

Error: Unable to find "/Users/mkonicek/Zero29App/package.json" or "/Users/mkonicek/Zero29App/node_modules/react-native/package.json". Make sure you ran "yarn" and that you are inside a React Native project.

Removed yarn.lock file, ran again:

Error: Unable to find "/Users/mkonicek/Zero29App/package.json" or "/Users/mkonicek/Zero29App/node_modules/react-native/package.json". Make sure you ran "npm install" and that you are inside a React Native project.

Success case:

Before, package.json contains:

"dependencies": {
  "react": "15.2.1",
  "react-native": "0.29.2"
}

Ran react-native-git-upgrade in project folder:

The app iupgr
Logs:

git-upgrade info Check for updates
git-upgrade info Using yarn 0.17.8
...
git-upgrade info Install the new version 
...
git-upgrade info Upgrade done

package.json contains:

"dependencies": {
  "react": "15.2.1",
  "react-native": "^0.38.0"
}

node_modules/react-native/package.json contains "version": "0.38.0".

Removed .yarn.lock file, the react-native-git-upgrade tool worked as before (without this PR).

Ran react-native-git-upgrade --help:

Usage: react-native-git-upgrade [version] [options]


Commands:

[Version]     upgrades React Native and app templates to the desired version
              (latest, if not specified)

Options:

-h, --help    output usage information
-v, --version output the version number
--verbose output debugging info
--npm force using the npm client even if your project uses yarn

The app upgraded from v0.29 to v0.38 runs:

[facebook-github-bot]

By analyzing the blame information on this pull request, we identified @ncuillery to be a potential reviewer.

[ncuillery]

LGTM

[mkonicek]

@facebook-github-bot shipit-ninja

[facebook-github-bot]

Normally, mkonicek, someone else should review and ship your pull request. However, we will trust that you know what you are doing. Shipping!

[facebook-github-bot]

@mkonicek has imported this pull request. If you are a Facebook employee, you can view this diff on Phabricator.