Update PackageCollectionModel.V1.ProductType init extension for .extension → .plugin renaming

Fix a build failure caused by SwiftPM upstream renaming `ProductType.extension` to `ProductType.plugin` in swiftlang/swift-package-manager#3311.

Note that SwiftPM's `PackageCollectionModel.V1` hasn't gotten this rename yet and still has `ProductType.extension`, which seems like a mismatch that probably needs to be resolved upstream (and then again here).

Author: friedbunny

Sources/PackageCollectionGenerator/PackageCollectionGenerate.swift

@@ -320,7 +320,7 @@ extension PackageCollectionModel.V1.ProductType {
             self = .library(.init(from: libraryType))
         case .executable:
             self = .executable
-        case .extension:
+        case .plugin:
             self = .extension
         case .test:
             self = .test