Add Swift Package comparator

[Kas]

Is your feature request related to a problem? Please describe.
As of Xcode 11, Xcode projects can now specify Swift Package dependencies for its targets.

Describe the solution you'd like
Add a comparator that can diff any declared Swift Packages, including version rules.

For example, given the following 2 projects:

Project 1:

Project 2:

Running xcdiff -v could produce results that look like:

❌ SWIFT_PACKAGES

⚠️  Only in second (1):

  • Swifter (https://github.com/httpswift/swifter)

⚠️  Value mismatch (1):

  • SQLite.swift - Version Rules
    ◦ Next Major From: 0.12.2
    ◦ Next Minor From: 0.12.2