Fix unknown target type in Swift 5.8

refs #803

`swift package resolve` in Swift 5.8.x:

```
Computing version for https://github.com/apple/swift-argument-parser.git
error: Invalid manifest (compiled with: ["/usr/bin/swiftc", "-vfsoverlay", "/tmp/TemporaryDirectory.HCuAi3/vfs.yaml", "-L", "/usr/lib/swift/pm/ManifestAPI", "-lPackageDescription", "-Xlinker", "-rpath", "-Xlinker", "/usr/lib/swift/pm/ManifestAPI", "-swift-version", "5", "-I", "/usr/lib/swift/pm/ManifestAPI", "-package-description-version", "5.8.0", "/Package@swift-5.8.swift", "-Xfrontend", "-disable-implicit-concurrency-module-import", "-Xfrontend", "-disable-implicit-string-processing-module-import", "-o", "/tmp/TemporaryDirectory.RLkqp3/swift-argument-parser-manifest"])
/Package@swift-5.8.swift:149:9: error: type 'Target.TargetType' has no member 'macro'
  case .macro, .plugin, .system, .binary:
       ~^~~~~ in https://github.com/apple/swift-argument-parser.git
```

Author: kphrx

Package@swift-5.8.swift

@@ -146,9 +146,7 @@ for target in package.targets {
     // https://github.com/swiftlang/swift-evolution/blob/main/proposals/0444-member-import-visibility.md
     settings.append(.enableUpcomingFeature("MemberImportVisibility"))
     target.swiftSettings = settings
-  case .macro, .plugin, .system, .binary:
+  default:
     break  // not applicable
-  @unknown default:
-    break  // we don't know what to do here, do nothing
   }
 }