Hide Miniz from public interface for library evolution support

Add @_implementationOnly to all Miniz imports to prevent the C module
from appearing in the public Swift interface, enabling use with
BUILD_LIBRARY_FOR_DISTRIBUTION.

Author: tomasf

Sources/Zip/ZipArchive+Get.swift

@@ -1,5 +1,5 @@
 import Foundation
-import Miniz
+@_implementationOnly import Miniz
 
 internal extension ZipArchive {
     func get<P: _Pointer>(_ body: (inout mz_zip_archive) -> P?) throws(ZipError) -> P {

Sources/Zip/ZipArchive+InMemory.swift

@@ -1,5 +1,5 @@
 import Foundation
-import Miniz
+@_implementationOnly import Miniz
 
 public extension ZipArchive<Data> {
 

Sources/Zip/ZipArchive+OnDisk.swift

@@ -1,5 +1,5 @@
 import Foundation
-import Miniz
+@_implementationOnly import Miniz
 
 public extension ZipArchive<URL> {
     /// Creates a `ZipArchive` instance for a disk-based zip archive at the specified file URL.

Sources/Zip/ZipArchive+Read.swift

@@ -1,5 +1,5 @@
 import Foundation
-import Miniz
+@_implementationOnly import Miniz
 
 public extension ZipArchive {
     /// Reads a file from the archive by its path.

Sources/Zip/ZipArchive+Write.swift

@@ -1,5 +1,5 @@
 import Foundation
-import Miniz
+@_implementationOnly import Miniz
 
 public extension ZipArchive {
     /// Adds a file to the archive with the specified data and path.

Sources/Zip/ZipArchive.swift

@@ -1,4 +1,4 @@
-import Miniz
+@_implementationOnly import Miniz
 import Foundation
 
 public class ZipArchive<Target> {

Sources/Zip/ZipError.swift

@@ -1,4 +1,4 @@
-import Miniz
+@_implementationOnly import Miniz
 import Foundation
 
 /// Swift representation of mz_zip_error