helm-source-file-cache doesn't read all files in file-cache-alist

The variable file-cache-alist is a list of (FILENAME DIRNAME1 DIRNAME2 ...),
but helm-source-file-cache reads only DIRNAME1/FILENAME.
The commit 482b55f18cc00e2fe210fb54754f11fdf3fdcba4 changes the behavior
of helm-source-file-cache.