Use Concat::File as type for $save_file dependency

Instead of depending on concat::file defining a file resource with the same name, depend on the concat::file resource directly.

We can safely do this, as the concat::file resource is defined in the same file (so we know that's the type used).

This change is useful for users of the iptables module that do not use the NSC concat module. In COS, we instead use the puppetlabs-concat module (as it supports showing diffs and doesn't store extra files in directories of managed files) with a compatibility shim to support Puppet code written for the NSC concat module. That module/shim combination does not define a file resource with the same name as the concat::file resource.