apple-xcframework: move headers into a Clibsodium subdirectory #807

Job	Run time
build-others	8m 45s
build-apple	15m 2s
build-windows-msvc	2m 27s
pack	40s
build-test-binaries	41s
run-test-binaries-os-versions (centos:8)	11s
run-test-binaries-os-versions (debian:10)	8s
run-test-binaries-cross-plat (arm)	43s
run-test-binaries-cross-plat (x64)	45s
run-test-binaries-cross-plat (arm64)	50s
	30m 12s

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