[cmake] more modules, general "cleanup" (#3126)

Successor to that old MoltenVK PR. Does a lot of cleanups within root CMakeLists.txt, hands over MoltenVK and VulkanValidationLayers to CPMUtil, and separates out common operations into my modules.

Hopefully reduces the monstrosity that is root CMakeLists.txt. Please test:

- builds on all platforms
- VulkanValidationLayers

Signed-off-by: crueter <crueter@eden-emu.dev>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/3126
Reviewed-by: Lizzie <lizzie@eden-emu.dev>
Reviewed-by: CamilleLaVey <camillelavey99@gmail.com>

externals/CMakeLists.txt

@@ -392,3 +392,17 @@ if (ANDROID)
 
     add_library(oboe::oboe ALIAS oboe)
 endif()
+
+# moltenvk
+if (NOT YUZU_USE_BUNDLED_MOLTENVK)
+    find_library(MOLTENVK_LIBRARY MoltenVK)
+else()
+    unset(MOLTENVK_LIBRARY)
+endif()
+
+# TODO: kosmickrisp?
+if (NOT MOLTENVK_LIBRARY)
+    AddJsonPackage(moltenvk)
+
+    set(MOLTENVK_LIBRARY "${moltenvk_SOURCE_DIR}/MoltenVK/dylib/macOS/libMoltenVK.dylib" CACHE STRING "" FORCE)
+endif()