Summary:
This diff supports monorepo/custom configuration where node_modules may not be located under app_root.
The default (RN_root, '..') should support most cases, but I also added an option to provide a custom location for users to set.
Changelog: [internal]
Reviewed By: ShikaSD
Differential Revision: D32469957
fbshipit-source-id: 6b8a6c775c21bde72ef542e34973701d698f678f