s:dobufread: use count with finddir

This function might need more fixes/changes, but using a count for now
seems to be good.

plug.vim

@@ -428,7 +428,7 @@ function! s:dobufread(names)
   for name in a:names
     let path = s:rtp(g:plugs[name]).'/**'
     for dir in ['ftdetect', 'ftplugin']
-      if len(finddir(dir, path))
+      if !empty(finddir(dir, path, 1))
         if exists('#BufRead')
           doautocmd BufRead
         endif