MaterialX does not follow symlinks in the library

[dgovil]

It appears that MaterialX does not follow directory symlinks when encountered under the libraries directory. I discovered this trying to symlink to a dev directory.

Steps to reproduce:

1. Build MaterialX somewhere
2. Go to the libraries folder and create a symlink to some other directory that contains a .mtlx file
3. Open the GraphEditor or list the nodes
4. See that anything in the symlinked directory is not included

Copying the file or symlinking the file directly seems to work. The issue just seems to be traversal of directories when the directory is a symlink.

This is on macOS