https://github.com/Shinmera/pathname-utils.git
git clone 'https://github.com/Shinmera/pathname-utils.git'
(ql:quickload :pathname-utils)
This is a collection of common tests and operations to help handling pathnames. It does not actually deal in handling the accessing of files on the underlying system however.
Since this is a purely utility/toolkit library, simply having a look at the symbol index should give you the best idea of what is offered.