PackageDescription: mark imports as implementation-only (#3228)

`Glibc`/`Darwin`/`ucrt` imports are not used in the public interface of `PackageDescription`, so there is no need to expose them.

This change hides the import statements from the generated `PackageDescription` module interface.

Author: egorzhdan

Sources/PackageDescription/PackageDescription.swift

@@ -9,12 +9,12 @@
 */
 
 #if canImport(Glibc)
-import Glibc
+@_implementationOnly import Glibc
 #elseif os(iOS) || os(macOS) || os(tvOS) || os(watchOS)
-import Darwin.C
+@_implementationOnly import Darwin.C
 #elseif os(Windows)
-import ucrt
-import struct WinSDK.HANDLE
+@_implementationOnly import ucrt
+@_implementationOnly import struct WinSDK.HANDLE
 #endif
 import Foundation