Mass-rename all relevant sources to a C++ extension

[pmatilai]

4.20 release is nigh (#3271) so as outlined in #3103, the time to do a mass-rename is closing in, this is simply a ticket to allow tracking + planning it. And maybe discuss a bit too.

The main "issue" here is deciding which suffix to use. From what I've seen, .cpp looks like the most common one for modern c++ code and would be in line with dnf5 which doesn't hurt either.

The other question is headers: we'll be dealing with plain C headers from here to eternity, so it seems like it'd be helpful to differentiate what is meant for C vs C++ consumption. .hpp seems like a consisntent and logical suffix there, and also what dnf5 uses. Not that it dictates in any way, but doesn't hurt either because it's a neighbor team really.