Support SwiftPM Package.pins

[andrew]

https://github.com/apple/swift-package-manager/blob/master/Documentation/Usage.md#package-pinning

JSON file called Package.pins which is generated/updated automatically as a lockfile

Only example I can find so far is here: https://github.com/apple/swift-package-manager/blob/master/Tests/WorkspaceTests/PinsStoreTests.swift#L99-L122

{
  "autoPin": true,
  "pins": [
    {
      "package": "bam",
      "reason": null,
      "repositoryURL": "/private/tmp/BigPackage/bam",
      "version": "1.0.0"
    },
    {
      "package": "bar",
      "reason": null,
      "repositoryURL": "/private/tmp/BigPackage/bar",
      "version": "1.0.0"
    },
    {
      "package": "baz",
      "reason": null,
      "repositoryURL": "/private/tmp/BigPackage/baz",
      "version": "1.0.0"
    }
  ],
  "version": 1
}

[andrew]

It's shipped in swift 3.1 so no longer on hold: kumabook/TOMLKit@6a7df51

[andrew]

Moving this to the Backlog as we'd still like to implement it but can't see that happening in the near future.