_CryptoExtras needs a CMakeLists.txt

[3405691582]

The toolchain bootstrap includes building swiftpm from cmake, and the swiftpm bootstrap also depends on swift-certificates and swift-crypto. The cmake bootstrap fails because swift-certificates refers to _CryptoExtras, and _CryptoExtras in swift-crypto has no CMakeLists.txt, so the module appears to be missing.

[Lukasa]

Yeah, this is a good catch. I'll try to find some time to get this done.