fix: fix @react-native-community/cli-platform-* packages not being found in monorepos

[tido64]

Summary:

Fix @react-native-community/cli-platform-* packages not being found in monorepos.

Note that we are making the assumption that process.cwd() returns the project root. This is the same assumption that @react-native-community/cli makes. Specifically, findProjectRoot() has an optional argument that defaults to process.cwd():

• findProjectRoot()
• Which gets called without arguments in loadConfig()
• loadConfig() gets called from setupAndRun(), also without project root set

As far as I can see, the project root argument is only ever used in tests.

Changelog:

[GENERAL] [FIXED] - Fix @react-native-community/cli-platform-* packages not being found in monorepos

Test Plan:

1. Clone/check out this branch: chore: bump react-native to 0.76 microsoft/rnx-kit#3409
2. Cherry-pick fix: fix @react-native-community/cli not being found in monorepos #47304
3. Cherry-pick fix: fix @react-native-community/cli-platform-* packages not being found in monorepos #47308
4. Run react-native config inside packages/test-app
5. Verify that projects is populated

Before:

  "healthChecks": [],
  "platforms": {},
  "assets": [],
  "project": {}
}

After:

  "healthChecks": [],
  "platforms": {
    "ios": {},
    "android": {}
  },
  "assets": [],
  "project": {
    "ios": {
      "sourceDir": "/~/packages/test-app/ios",
      "xcodeProject": {
        "name": "SampleCrossApp.xcworkspace",
        "path": ".",
        "isWorkspace": true
      },
      "automaticPodsInstallation": false,
      "assets": []
    },
    "android": {
      "sourceDir": "/~/packages/test-app/android",
      "appName": "app",
      "packageName": "com.msft.identity.client.sample.local",
      "applicationId": "com.msft.identity.client.sample.local",
      "mainActivity": "com.microsoft.reacttestapp.MainActivity",
      "assets": []
    }
  }
}