Add Swift Package Registry support

[albertodebortoli]

These changes allow the generation of Package.swift files when using a Swift Registry.

The dependency in the dependencies file should have an identifier value. The url value is not needed when using a registry but it's kept for backward compatibility (it would also come handy for documentation purposes to re).

dependencies:
  - name: RemoteDependency
    identifier: acme.RemoteDependency
    url: https://github.com/RemoteDependency
    version: 1.0.0

The Spec file will have to specify which remote dependency is referenced from the target dependencies to resolve the correct value for the package parameter (the identifier) in the product definition (.product(name: "RemoteDependency", package: "acme.RemoteDependency")). We cannot just rely on the name value since it might differ from what used in the dependencies file, as expressed in the test using the _SDK suffix.

name: Registry
products:
  - name: Registry
    productType: library
    targets:
      - Target
localDependencies: []
remoteDependencies:
  - name: RemoteDependency
targets:
  - name: Target
    targetType: target
    dependencies:
      - name: RemoteDependency_SDK
        dependency: RemoteDependency
    sourcesPath: Framework/Sources
    resourcesPath: Resources

Reflecting as follows in the Package.swift file:

...
let package = Package(
    ...
    dependencies: [
        .package(
            id: "acme.RemoteDependency",
            exact: "2.0.0"
        ),
    ],
    targets: [
        .target(
            name: "Target",
            dependencies: [
                .product(name: "RemoteDependency_SDK", package: "acme.RemoteDependency")
            ],
            ...
        )
    ]
)

[akuzminskyi]

out of the scope, 6.1 is available