Fix preconcurrency-related warnings (#5907)

Since we merged swiftlang/swift-tools-support-core#367

Author: neonichu

Examples/package-info/Sources/package-info/example.swift

@@ -1,10 +1,9 @@
 #if swift(>=5.7)
 @preconcurrency import Basics
-@preconcurrency import TSCBasic
 #else
 import Basics
-import TSCBasic
 #endif
+import TSCBasic
 import Workspace
 
 @main

Tests/BasicsTests/TemporaryFileTests.swift

@@ -9,11 +9,7 @@
  */
 
 import XCTest
-#if swift(>=5.7)
-@preconcurrency import TSCBasic
-#else
 import TSCBasic
-#endif
 
 import Basics