Extend SwiftPM `PackageDescription` to introduce metadata

[mattt]

I agree that we should have a conventional way to represent package information. However, I think it'd be a mistake to invest in a new, Swift-specific solution.

Schema.org provides a comprehensive standard for representing project metadata through its SoftwareSourceCode data type.

As discussed in our pitch for a Swift package registry specification:

@mattt

A server MAY include additional fields in its response. It is RECOMMENDED that package metadata be represented in JSON-LD according to a structured data standard. For example, this response using the Schema.org SoftwareSourceCode vocabulary:

{
  "@context": ["http://schema.org/"],
  "@type": "SoftwareSourceCode",
  "identifier": "@mona/LinkedList",
  "name": "LinkedList",
  "description": "One thing links to another.",
  "keywords": ["data-structure", "collection"],
  "version": "1.1.1",
  "codeRepository": "https://github.com/mona/LinkedList",
  "license": "https://www.apache.org/licenses/LICENSE-2.0",
  "readme": "https://github.com/mona/LinkedList/blob/master/README.md",
  "issueTracker": "https://github.com/mona/LinkedList/issues",
  "programmingLanguage": {
    "@type": "ComputerLanguage",
    "name": "Swift",
    "url": "https://swift.org"
  },
  "author": {
      "@type": "Person",
      "@id": "https://github.com/mona",
      "givenName": "Mona",
      "middleName": "Lisa",
      "familyName": "Octocat"
  }
}

By representing project metadata as linked data, it can be understood by machines and integrated with other linked data. Also, the specific choice of encoding is less important; the most obvious choice here would be JSON-LD, but the same information could be represented as RDF.

We could spend a lot of time coming up with our own solution here, but I don't think that we'd end up with anything more capable than the Schema.org definition.