draft: CPack/DEB: parallel parameter generation

This is a very incomplete draft of the CPackDEB C++ rewrite discussed in !8595 (closed)

Work left to be done:

• Actually finish porting CPackDeb.cmake
• Use libuv to execute all of these state machines in parallel

I'm mostly looking for feedback on the general design. I haven't even tried to build my new code yet as it's clearly nonfunctional.

Some notes/questions:

• I'm not sure if it's worth using libuv for all the subprocesses we need to run. This initial version shows preempting for the file commands--maybe that just makes the code more difficult to read for little reason. We could potentially only preempt for calls that are measured to be slow.