devtool: Fix file:// fetcher symlink directory structure

Add relative path if file is under directory.

[YOCTO #13738]

(From OE-Core rev: 19ddacc1)

Signed-off-by: Tomasz Dziendzielski <tomasz.dziendzielski@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 3a220f1e411767cbf9e7099c18d94a12171c1093)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>