Ignore directories in the transfer plugin

[bbockelm]

HTCSS, in certain configurations, ask Pelican to transfer a directory on stageout. This is incorrect -- HTCSS already iterates through the directory and provides a list of objects.

While we are waiting on a fix from upstream, we should simply skip requests to upload a directory; that is, stat the name being given to the plugin and continue in the loop if the name refers to a directory.