SE-0540: Default Package Settings

[ksluder (Kyle Sluder)]

@xwu

That is, let these settings be declared as top-level let myDefaultSwiftSettings = [ /* defaults */ ] and then write .target(name: "foo", swiftSettings: myDefaultSwiftSettings + [ /* target-specific settings */ ]).

I still strongly believe it would be a mistake to continue to rely on Swift package manifests being executable Swift code.