Add explicit access-level modifier support for imports in code generator (#2010)

## Motivation
This is a follow-up of https://github.com/grpc/grpc-swift/pull/2003.
As of 5.9, Swift supports access-level modifiers on imports. In an
upcoming Swift 6.x release, the default modifier will become `internal`.

## Modifications
This PR adds explicit access-level modifiers to the generated code and a
few other modules that use this generated code.

## Result
Explicit access level imports present in more places.

Tests/GRPCHTTP2TransportTests/ControlService.swift

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-import GRPCCore
+internal import GRPCCore
 
 import struct Foundation.Data
 

Tests/GRPCHTTP2TransportTests/Generated/control.grpc.swift

@@ -22,8 +22,8 @@
 // For information on using the generated types, please see the documentation:
 //   https://github.com/grpc/grpc-swift
 
-import GRPCCore
-import GRPCProtobuf
+internal import GRPCCore
+internal import GRPCProtobuf
 
 internal enum Control {
     internal static let descriptor = ServiceDescriptor.Control

Tests/GRPCHTTP2TransportTests/HTTP2TransportNIOPosixTests.swift

@@ -14,10 +14,10 @@
  * limitations under the License.
  */
 
-import GRPCCore
-import GRPCHTTP2Core
-import GRPCHTTP2TransportNIOPosix
-import XCTest
+private import GRPCCore
+private import GRPCHTTP2Core
+private import GRPCHTTP2TransportNIOPosix
+internal import XCTest
 
 @available(macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0, *)
 final class HTTP2TransportNIOPosixTests: XCTestCase {

Tests/GRPCHTTP2TransportTests/HTTP2TransportNIOTransportServicesTests.swift

@@ -15,10 +15,10 @@
  */
 
 #if canImport(Network)
-import GRPCCore
-import GRPCHTTP2Core
-import GRPCHTTP2TransportNIOTransportServices
-import XCTest
+private import GRPCCore
+private import GRPCHTTP2Core
+internal import GRPCHTTP2TransportNIOTransportServices
+internal import XCTest
 
 @available(macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0, *)
 final class HTTP2TransportNIOTransportServicesTests: XCTestCase {

Tests/GRPCHTTP2TransportTests/HTTP2TransportTests.swift

@@ -14,12 +14,12 @@
  * limitations under the License.
  */
 
-import GRPCCore
-import GRPCHTTP2Core
-import GRPCHTTP2TransportNIOPosix
-import GRPCHTTP2TransportNIOTransportServices
-import GRPCProtobuf
-import XCTest
+internal import GRPCCore
+private import GRPCHTTP2Core
+private import GRPCHTTP2TransportNIOPosix
+private import GRPCHTTP2TransportNIOTransportServices
+private import GRPCProtobuf
+internal import XCTest
 
 @available(macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0, *)
 final class HTTP2TransportTests: XCTestCase {

Tests/GRPCHTTP2TransportTests/Test Utilities/HTTP2StatusCodeServer.swift

@@ -14,11 +14,11 @@
  * limitations under the License.
  */
 
-import GRPCHTTP2Core
-import NIOCore
-import NIOHPACK
-import NIOHTTP2
-import NIOPosix
+internal import GRPCHTTP2Core
+private import NIOCore
+private import NIOHPACK
+private import NIOHTTP2
+private import NIOPosix
 
 /// An HTTP/2 test server which only responds to request headers by sending response headers and
 /// then closing. Each stream will be closed with the ":status" set to the value of the

Tests/GRPCHTTP2TransportTests/Test Utilities/XCTest+Utilities.swift

@@ -13,8 +13,8 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-import GRPCCore
-import XCTest
+
+private import XCTest
 
 func XCTAssertThrowsError<T, E: Error>(
   ofType: E.Type,

Tests/GRPCHTTP2TransportTests/XCTestCase+Vsock.swift

@@ -14,8 +14,8 @@
  * limitations under the License.
  */
 
-import NIOPosix
-import XCTest
+private import NIOPosix
+internal import XCTest
 
 extension XCTestCase {
   func vsockAvailable() -> Bool {