Discover symlinked scoped dependencies in findSymlinksPaths

[alexhanson]

This builds upon the work by @harshil07 (thank you!) in pull request #11810, adding support for symlinked scoped dependencies (like @foo/bar). Now, when we see an @-prefixed folder, we'll look inside it for symlinks.

If a project isn't using scoped dependencies, this change doesn't cause any additional file system operations.

Test plan

1. Create a symlink like @foo/bar in your project's node_modules folder (e.g., by using npm link or yarn link). @foo will be a real folder, but bar will be a symlink.
2. Run react-native bundle, react-native run-ios, or another command that will explore your dependency graph.

Code formatting
Please let me know if there's anything you'd like me to address!

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[alexhanson]

Closing this pull request given that #12435 was accepted in commit c97c1e5.

[alexhanson]

Contrary to my comment above, #12435 addressed a different issue than this pull request. However, #12400 does address the issue I described here, and it remains open. Anybody having issues with symlinked scoped dependencies should look at #12400 instead.