Ease binary distribution: testing

Add a SYSTEM_PACKAGE define and conditionalize import statements in tests to
ease testing of binary distributions of System.

Author: milseman

Package.swift

@@ -29,7 +29,10 @@ let targets: [PackageDescription.Target] = [
 
   .testTarget(
     name: "SystemTests",
-    dependencies: ["SystemPackage"]),
+    dependencies: ["SystemPackage"],
+    swiftSettings: [
+      .define("SYSTEM_PACKAGE")
+    ]),
 ]
 
 let package = Package(

Tests/SystemTests/ErrnoTest.swift

@@ -8,7 +8,12 @@
 */
 
 import XCTest
+
+#if SYSTEM_PACKAGE
 import SystemPackage
+#else
+import System
+#endif
 
 // @available(macOS 10.16, iOS 14.0, watchOS 7.0, tvOS 14.0, *)
 final class ErrnoTest: XCTestCase {

Tests/SystemTests/FileOperationsTest.swift

@@ -8,7 +8,12 @@
 */
 
 import XCTest
+
+#if SYSTEM_PACKAGE
 import SystemPackage
+#else
+import System
+#endif
 
 // @available(macOS 10.16, iOS 14.0, watchOS 7.0, tvOS 14.0, *)
 final class FileOperationsTest: XCTestCase {

Tests/SystemTests/FilePathTests/FilePathComponentsTest.swift

@@ -8,10 +8,12 @@
 */
 
 import XCTest
-import SystemPackage
 
-@testable
-import SystemPackage
+#if SYSTEM_PACKAGE
+@testable import SystemPackage
+#else
+@testable import System
+#endif
 
 // @available(9999....)
 struct TestPathComponents: TestCase {

Tests/SystemTests/FilePathTests/FilePathExtras.swift

@@ -1,5 +1,9 @@
 
+#if SYSTEM_PACKAGE
 @testable import SystemPackage
+#else
+@testable import System
+#endif
 
 // Why can't I write this extension on `FilePath.ComponentView.SubSequence`?
 extension Slice where Base == FilePath.ComponentView {

Tests/SystemTests/FilePathTests/FilePathParsingTest.swift

@@ -8,7 +8,12 @@
 */
 
 import XCTest
+
+#if SYSTEM_PACKAGE
 import SystemPackage
+#else
+import System
+#endif
 
 private struct ParsingTestCase: TestCase {
   // Whether we want the path to be constructed and syntactically
@@ -57,7 +62,11 @@ extension ParsingTestCase {
   }
 }
 
+#if SYSTEM_PACKAGE
 @testable import SystemPackage
+#else
+@testable import System
+#endif
 
 final class FilePathParsingTest: XCTestCase {
   func testNormalization() {

Tests/SystemTests/FilePathTests/FilePathSyntaxTest.swift

@@ -8,7 +8,12 @@
 */
 
 import XCTest
+
+#if SYSTEM_PACKAGE
 import SystemPackage
+#else
+import System
+#endif
 
 private struct SyntaxTestCase: TestCase {
   // Whether we want the path to be constructed and syntactically

Tests/SystemTests/FilePathTests/FilePathTest.swift

@@ -8,7 +8,12 @@
 */
 
 import XCTest
+
+#if SYSTEM_PACKAGE
 import SystemPackage
+#else
+import System
+#endif
 
 // @available(macOS 10.16, iOS 14.0, watchOS 7.0, tvOS 14.0, *)
 func filePathFromUnterminatedBytes<S: Sequence>(_ bytes: S) -> FilePath where S.Element == UInt8 {

Tests/SystemTests/FileTypesTest.swift

@@ -8,7 +8,12 @@
 */
 
 import XCTest
+
+#if SYSTEM_PACKAGE
 import SystemPackage
+#else
+import System
+#endif
 
 // @available(macOS 10.16, iOS 14.0, watchOS 7.0, tvOS 14.0, *)
 final class FileDescriptorTest: XCTestCase {

Tests/SystemTests/MockingTest.swift

@@ -8,7 +8,13 @@
 */
 
 import XCTest
+
+#if SYSTEM_PACKAGE
 import SystemPackage
+#else
+import System
+#endif
+
 @testable import SystemInternals
 
 // @available...