Fix bug in symlink traversal with indirect targets

[robhogan]

Summary: Fixes a bug where symlinks beginning with indirections (../) may fail to traverse in some projectRoot / watchFolders configurations.

This occurs due to incomplete "normalisation" of a symlink target in metro-file-map, relative to the project root. Given a project root of /project and a symlink /project/src/link-to-foo -> ../../project/foo.js, we would normalise the target to ../project/foo.js rather than simply foo.js - we were not taking into account that further normalisation could be possible after joining with projectRoot.

"Normal" paths are defined as being fully normalised relative to the project root. The unnecessary indirection out and back into the project root fails because TreeFS is a DAG, and internally there is no entry for rootNode.get('..').get('project') - this would be a cycle back to rootNode.

Changelog:

 * **[Fix]:** Symlinks with indirections may not be resolvable

Differential Revision: D49323614

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D49323614

[facebook-github-bot]

This pull request has been merged in facebook/metro@6f3f7e681a3f400c2332b92105812339e4dbf1f4.