Tests: remove @testable import PackageFingerprint

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

Author: compnerd

Tests/PackageFingerprintTests/FilePackageFingerprintStorageTests.swift

@@ -13,7 +13,7 @@
 import Basics
 import _Concurrency
 import struct Foundation.URL
-@testable import PackageFingerprint
+import PackageFingerprint
 import PackageModel
 import _InternalTestSupport
 import XCTest