Fix mapping of registry SwiftPackageManager dependencies

[danieleformichelli]

#5398

Short description 📝

Looking at the code this should work, but I have not been able to properly define a registry dependency to test it

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[regularberry]

@danyf90 I don't have a sample project, but I can provide some example output when I point a package at our internal registry.
I set one up for swift-algorithms.

Package.swift

// swift-tools-version: 5.8
import PackageDescription

let package = Package(
    name: "PackageName",
    dependencies: [
        .package(
            id: "sean-test.swift-algorithms",
            from: "1.0.0"
         ),
    ]
)

Package.resolved

{
  "pins" : [
    {
      "identity" : "sean-test.swift-algorithms",
      "kind" : "registry",
      "location" : "",
      "state" : {
        "version" : "1.0.0"
      }
    },
    {
      "identity" : "swift-numerics",
      "kind" : "remoteSourceControl",
      "location" : "https://github.com/apple/swift-numerics.git",
      "state" : {
        "revision" : "0a5bc04095a675662cf24757cc0640aa2204253b",
        "version" : "1.0.2"
      }
    }
  ],
  "version" : 2
}

workspace-state.json

{
  "object" : {
    "artifacts" : [

    ],
    "dependencies" : [
      {
        "basedOn" : null,
        "packageRef" : {
          "identity" : "sean-test.swift-algorithms",
          "kind" : "registry",
          "location" : "sean-test.swift-algorithms",
          "name" : "sean-test.swift-algorithms"
        },
        "state" : {
          "name" : "registryDownload",
          "version" : "1.0.0"
        },
        "subpath" : "sean-test/swift-algorithms/1.0.0"
      },
      {
        "basedOn" : null,
        "packageRef" : {
          "identity" : "swift-numerics",
          "kind" : "remoteSourceControl",
          "location" : "https://github.com/apple/swift-numerics.git",
          "name" : "swift-numerics"
        },
        "state" : {
          "checkoutState" : {
            "revision" : "0a5bc04095a675662cf24757cc0640aa2204253b",
            "version" : "1.0.2"
          },
          "name" : "sourceControlCheckout"
        },
        "subpath" : "swift-numerics"
      }
    ]
  },
  "version" : 6
}

I tried the commit in this PR, but it crashes due to the provided subpath sean-test/algorithms/1.0.0 not pointing to anything.

I think the solution could be to just skip it for the registry type, since it adds a second entry in Package.resolved.

So:

case "registry":
    return nil

And then change the .map to a .compactMap. This worked for me locally!

[danieleformichelli]

Returning null there it would ignore the swift-algorithms package entirely and just import swift-numeric (its dependency) 🤔
What's the path on disk of that inside Tuist/Dependencies/SwiftPackageManager/.build folder?