🔧 Allow superproject to provide dependencies with add_subdirectory()

[thirtythreeforty]

This lets a superproject vendor cpp-yyjson along with these dependencies, doing something like:

add_subdirectory(yyjson)
add_subdirectory(fmt)
add_subdirectory(nameof)
add_subdirectory(cpp-yyjson)

and cpp-yyjson will use the dependencies provided by the parent Cmake project, without trying to find them installed.

[yosh-matsuda]

Merged, thanks!

[thirtythreeforty]

Thanks! And thanks for the second pass, I overlooked the versioning checks.