CMake build does follow patterns from other swift-* projects (e.g. collections) #59

When recently consuming swift-atomics and swift-collections I noticed a weird difference in where cmake files end up:

-> % find .build/checkouts/swift-atomics | grep onfig.cmake
.build/checkouts/swift-atomics/swift-atomics-config.cmake


-> % find .build/checkouts/swift-collections | grep onfig.cmake
.build/checkouts/swift-collections/cmake/modules/SwiftCollectionsConfig.cmake
.build/checkouts/swift-collections/cmake/modules/SwiftCollectionsConfig.cmake.in

probably the files in atomics should also end up in cmake/modules rather than in top level.
This made consuming swift-atomics a bit confusing; though everything works, it might be nicer to follow the same patterns in our packages.