Tests: remove @testable import PackageLoading

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

Author: compnerd

Tests/PackageLoadingTests/ManifestLoaderCacheTests.swift

@@ -11,7 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 import Basics
-@testable import PackageLoading
+import PackageLoading
 import PackageModel
 import _InternalTestSupport
 import XCTest

Tests/PackageLoadingTests/PkgConfigParserTests.swift

@@ -11,7 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 import Basics
-@testable import PackageLoading
+import PackageLoading
 import _InternalTestSupport
 import XCTest
 

Tests/PackageLoadingTests/PkgConfigTests.swift

@@ -11,7 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 import Basics
-@testable import PackageLoading
+import PackageLoading
 import PackageModel
 import _InternalTestSupport
 import XCTest