refine regex for \graphicspath argument

[dginev]

Fixes #2159

Tested with

\documentclass{article}
\usepackage{graphicx}
\graphicspath{  {subdir1/}, test/,  {subdir2/}  {subdir3/}

{subdirn/}  }
\begin{document}
\end{document}

Result:

<?xml version="1.0" encoding="UTF-8"?>
<?latexml searchpaths="/home/deyan/git/my-LaTeXML,/tmp"?>
<?latexml class="article"?>
<?latexml package="graphicx"?>
<?latexml graphicspath="/tmp/subdir1"?>
<?latexml graphicspath="/tmp/test"?>
<?latexml graphicspath="/tmp/subdir2"?>
<?latexml graphicspath="/tmp/subdir3"?>
<?latexml graphicspath="/tmp/subdirn"?>
<?latexml RelaxNGSchema="LaTeXML"?>
<document xmlns="http://dlmf.nist.gov/LaTeXML">
  <resource src="LaTeXML.css" type="text/css"/>
  <resource src="ltx-article.css" type="text/css"/>
</document>

[brucemiller]

Cool; Thanks!