Improve Qt5 detection on macOS

[tresf]

Supersedes:

• Add better Qt5 detection #4067

Originally, the intention in #4067 was to fix Qt detection for all platforms, however due to a multitude of reasons, I don't think that's practical due to:

1. The variety of Qt installation locations and types on Windows (e.g. mingw, msvc, etc)
2. The changes in how deliberately difficult Qt has made it to obtain Qt > 5.14 standalone on Linux
3. The fact that 99.9% of Linux devs will just use what their system provides, which is already detecting properly.

So, instead, I'll focus on macOS, that's historically always had to use CMAKE_PREFIX_PATH=$(brew --prefix qt@5) (which as a macOS user, I'm always forgetting to type before running cmake).

I've chosen to leverage Qt5_DIR and Qt5Test_DIR for simplicity, since the shell commands add much more conditional logic that's not necessarily warranted.

I've added future support for Qt6.

[tresf]

I'm squashing this so that I can cherry-pick it atop #7339 for testing.

[tresf]

Qt5 and Qt6 both work out of the box now. I'll attempt MacPorts as well, but if I hit too many unrelated snags, I'll just merge.

Testing:

• Homebrew + Qt5 (aarch64)
• Homebrew + Qt6 (aarch64)
• MacPorts + Qt5 (aarch64)
• MacPorts + Qt6 (aarch64)