Let TeXFiles.py handle symbolic links.

Let TeXFiles.py handle symbolic links.

Nix (https://nixos.org) is a Unix package manager, which can be used to
install LaTeX on macOS. A peculiarity of Nix is that all packages are
installed into separate directories and the actual directory tree is
then constructed via symlinks.

This interacts badly with the way LyX currently detects files in the
TeX setup, because TeXFiles.py does not follow symlinks. Therefore,
almost nothing is found when using LyX together with Nix’ LaTeX.

Patch from Michael Roitzsch.

(cherry picked from commit 642b4acca1900c55662030ffc5162e9504881764)