source_group: Fix regression in relative FILES
Fix the check added in !3265 (merged) to convert to an absolute path before checking for existence.
Also simplify the conversion to an absolute path.
Fixes: #19454 (closed)
Fix the check added in !3265 (merged) to convert to an absolute path before checking for existence.
Also simplify the conversion to an absolute path.
Fixes: #19454 (closed)