Rather than set the FOLDER property from within certain third-party
CMakeLists.txt, do so from the root, and make them all go to a
"third_party" folder.

Bug: b/145758253
Change-Id: I7b9f69a8f16be8ef6e0d153fc28ac4f6c3d64e5e
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/48368
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>