devtool: Fix file:// fetcher symlink directory structure

Add relative path if file is under directory.

[YOCTO #13738]

Signed-off-by: Tomasz Dziendzielski <tomasz.dziendzielski@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 19ddacc1)
Signed-off-by: Steve Sakoman <steve@sakoman.com>