Extend SwiftPM `PackageDescription` to introduce metadata

[SDGGiesbrecht (Jeremy David Giesbrecht)]

Some prior discussion exists here:

[Pitch] Adding metadata to SPM Package

Hi, per a suggestion from @ddunbar I'd like to pitch the idea of adding a .metadata category to SPM Package definition. The motivation is to support the (eventual) expansion of the use of SPM for more user-facing targets with the addition of SwiftUI, Xcode 11 support for SPM, etc. This follows common practice with other packaging managers such as gems, npm, and pod specs. Further, I would like to pitch a Swifty version of this metadata, something like the following would be ideal for our use ca…