Provide information to metro to redirect to rn-macos when building macos

[acoates-ms]

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native-macos 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native-macos 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

Summary

In react-native-community/cli#1115 I added support to RN cli to modify the default configuration of metro to support out of tree platforms. This requires that out of tree platforms specify an additional peice of configuration in their react-native.config.js which RN-macOS isn't currently doing.

This is likely the issue with #575 and #564. We'll want to backport this to 0.62 too.

Microsoft Reviewers: Open in CodeFlow

[pull-bot]

	Messages
📖	📋 Missing Changelog - Can you add a Changelog? To do so, add a "## Changelog" section to your PR description. A changelog entry has the following format: [CATEGORY] [TYPE] - Message. CATEGORY may be: General macOS iOS Android JavaScript Internal (for changes that do not need to be called out in the release notes) TYPE may be: Added, for new features. Changed, for changes in existing functionality. Deprecated, for soon-to-be removed features. Removed, for now removed features. Fixed, for any bug fixes. Security, in case of vulnerabilities. MESSAGE may answer "what and why" on a feature level. Use this to briefly tell React Native users about notable changes.
📖	📋 Missing Test Plan - Can you add a Test Plan? To do so, add a "## Test Plan" section to your PR description. A Test Plan lets us know how these changes were tested.

Generated by 🚫 dangerJS against a9ac953

[Abhishek12345679]

can anyone let me know why did it fail ?

[tom-un]

can anyone let me know why did it fail ?

There are intermittent failures during yarn install in the tests. I re-ran it and it passed.