relates to:

• SR-2917 Handle different URLs referring same package

Issue Description:

Currently if you define a package like this:

import PackageDescription

let package = Package(
    name: "Sonar-Example",

    dependencies: [
       .Package(url: "../", majorVersion: 0),
    ]
)

The Package.pins is stored as:

{
  "autoPin": true,
  "pins": [
    {
      "package": "Sonar",
      "reason": null,
      "repositoryURL": "/Users/ksmiley/dev/Sonar",
      "version": "0.1.0"
    }
  ],
  "version": 1
}

This isn't very portable for packages where you're using relative paths either for an example project, or with submodules. It would be nice if the pins file stored the original relative path (`../`) so that it would work on other clones.