Add function to get file source from file struct, and use it instead of file_exists.
kernel: Refactor code to use single function to check if modules.dep exists.

And don't use file_exists.
package handlers: Remove use of file_exists function (RHBZ#1082044).

Use equivalent code using stat.

On CentOS this should detect /etc/redhat-release symlink.
Remove generic 'file_exists' function from utils.ml.
Version 5.1.7.