Establish uniformity across targets in the way files are looked up

[lhstrh]

I believe that the findFile method implements the policy all code generators should use for locating files that are either imported or listed in the files target property. If they don't, we should bring all targets in alignment, and if they do we have a case of code duplication. We might create an issue for this and move on?

I think currently we don't support the files property outside the C-based targets. I am not sure how useful it would be for the other targets. For C++, it only adds redundancy but no new functionality. But yes, we can and should discuss this elsewhere.

Originally posted by @cmnrd in #978 (comment)