Tests: remove @testable import PackageRegistryCommand

Use the newly extended APIs for testing and use a non-testable import of
the package, avoiding a recompilation.

Author: compnerd

Tests/CommandsTests/PackageRegistryCommandTests.swift

@@ -16,7 +16,7 @@ import Foundation
 import PackageLoading
 import PackageModel
 import PackageRegistry
-@testable import PackageRegistryCommand
+import PackageRegistryCommand
 import PackageSigning
 import _InternalTestSupport
 import TSCclibc // for SPM_posix_spawn_file_actions_addchdir_np_supported