I'm not sure. Maybe the correct thing to do is the opposite - consolidate all AUTHORS and CONTRIBUTORS to the repository root. The same for license files.
If you're fine with the idea of merging all the files in question to the repo root: I can do that or you can send a MR if you prefer.
Wrt to packaging different versions: cc/v3 is more or less usable, but I personally consider cc/v4 the only version worth packaging. (v5 is a different story, it's @dennwc's fork of v4, but in the same repo.)
However, cc/v4 is nowadays being updated several times a week. Not sure if that plays well with packaging.
Out of curiosity: How does packaging of Go packages work? When someone does import modernc.org/cc/v4 does the go tool on Fedora first look for the locally installed distro package, if any?