MaterialX does not follow symlinks in the library

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

Start by tracing the library-directory traversal used when the GraphEditor opens libraries or nodes are listed, then reproduce the macOS case with a directory symlink containing a .mtlx file. Done means files inside the symlinked directory are included while direct file symlinks continue to work.

Written by the indexing model from the issue text.