Add support for Swift PM

[pombredanne]

See https://swift.org/package-manager/

• https://github.com/mapbox/mapbox-maps-ios/blob/main/Package.swift
• https://github.com/mapbox/mapbox-maps-ios/blob/main/Package.resolved

See also some references:

• https://github.com/klassen-software-solutions/LicenseScanner/blob/master/kss/license/swift_scanner.py
• https://github.com/DataDog/dd-sdk-ios/blob/8673c7a076a86838bbe9294de4ee1f3feb9ea9b7/tools/dogfooding/src/package_resolved.py
• https://github.com/readdle/swift-android-buildtools/tree/4df7dc89b0d8e1817484bedfe92e3fed67e15423/src/python

[pombredanne]

There are a few use cases:

• we have a Package.resolved (solo or with a Package.swift source)
• we do not have a Package.resolved, only Package.swift

We would need to run the swiftpm with either options:

• swift package show-dependencies
• swift package dump-package

And get a JSON output.
Running these commands should be only documented for now. The general approach will be tracked in this issue to run package managers outside of ScanCode.

• DEPLOCK: Create initial Lockfile checker and generator  dependency-inspector#2

[pombredanne]

A twist to consider later: https://docs.swift.org/package-manager/PackageDescription/PackageDescription.html#package-dependency

If you add the Swift package as a package dependency to an app for an Apple platform, you can find the Package.resolved file inside your .xcodeproj or .xcworkspace.