Cmake cleanups

[d235j]

This is a heavy refactor of the Cmake scripts to increase the use of autodetection and reduce the reliance on FindVulkan, which is going away in the future and is unreliable when the various components are installed individually.

This requires the related PR for scopehal, tracked at https://github.com/ngscopeclient/scopehal/pull/841.

This is ready for merge; updates to the docs will be needed though.

• [x] Test on macOS arm64 (individually installed components)
• [x] Test on macOS arm64 (Vulkan SDK)
• [x] Test on macOS x86_64 (individually installed components)
• [x] Test on macOS x86_64 (Vulkan SDK)
• [x] Test on Linux x86_64 (Arch Linux, individually installed components)
• [x] Test on Linux x86_64 (Debian oldstable, Vulkan SDK from tarball)
• [x] Test on Linux x86_64 (Debian oldstable, individual components from Vulkan SDK repo [unsupported config but occasionally used])
• [x] Test on Linux x86_64 (Debian stable, Vulkan SDK)
• [x] Test on Linux x86_64 (Debian stable, individually installed components)
• [x] Test on Linux x86_64 (Fedora current, Vulkan SDK)
• [x] Test on Linux x86_64 (Fedora current, individually installed components) - [ ] Test on Windows MINGW64 (Vulkan SDK) UNSUPPORTED due to C++ linkage differences
• [x] Test on Windows MINGW64 (individually installed components)

[d235j]

Seems to work on most platforms; needs a bit more testing on Windows. macOS x86_64 not yet tested either.

[d235j]

This was merged together with #684.