CPack/IFW: Add missing referenced source file validation
A few file names used as input were not being validated (some new for 3.23, some existing). I've prepared this as a separate change so it can be included in the 3.23.0 release, or it can be pushed back to a 3.23.1 patch release if you think it's too deep into the current release cycle to put in a code change like this one.
Backport: release