Use either Git submodules (preferable) or CMake FetchContent

[OPNA2608]

There is a Git submodule (dependency fetching mechanism # 1) for JUCE that gets semi-regularly updated, but it's completely ignored by the CMake script which uses FetchContent (dependency fetching mechanism # 2) for JUCE and the other required dependencies instead.

Mixing these is weird because the submodule is essentially never used by CMake, it seems like a mistake? For me at least (trying to package this in a Linux/macOS package manager) it would be preferable to only use Git submodules and replace the FetchContent_* calls with add_subdirectory(<dependency> EXCLUDE_FROM_ALL) ones, because FetchContent isn't very compatible with the way we do fetching.