_CryptoExtras needs a CMakeLists.txt

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.

Start by inspecting the _CryptoExtras module and the CMake bootstrap configuration, then run the failing bootstrap to confirm how swift-certificates refers to it. Done means _CryptoExtras is included in the CMake build and the swiftpm bootstrap no longer fails because the module appears missing.

Written by the indexing model from the issue text.