Add Linux support

[zoul]

Running tests through Docker:

docker run --rm --volume "$(pwd):/package" --workdir "/package" swift:4.2 /bin/bash -c "swift test"

• Make swift test --generate-linuxmain a part of the build process so that the list of tests for Linux never gets out of sync?
• Make the Linux build & test part of the CI build
• Change the repo layout to the standard SPM one?
• Contribute a better error message for BuildPlan.missingLinuxMain to SPM?