Use .enableUpcomingFeature() for StrictConcurrency and InternalImportsByDefault.

Both the `StrictConcurrency` and `InternalImportsByDefault` features became
"upcoming" features instead of "experimental" features in the Swift 6 compiler.
This means that specifying those features with `.enableExperimentalFeature()`
no longer works, and therefore these settings in SwiftPM's Package.swift have
become inactive. I'm making the compiler more lenient, so that
`-enable-experimental-feature` will enable the corresponding upcoming feature
if relevant (swiftlang/swift#75962) but in the meantime
it appears that some of the code in SwiftPM needs some fixes to continue to be
accepted by the compiler with these features on.

Additionally, I've removed explicit enablement of `AccessLevelOnImports` since
that feature became standard in the Swift 5.10 compiler.

Author: tshortli

Package.swift

@@ -171,7 +171,6 @@ let package = Package(
             ],
             exclude: ["CMakeLists.txt"],
             swiftSettings: [
-                .enableExperimentalFeature("AccessLevelOnImport"),
                 .unsafeFlags(["-static"]),
             ]
         ),
@@ -187,9 +186,8 @@ let package = Package(
             ],
             exclude: ["CMakeLists.txt"],
             swiftSettings: [
-                .enableExperimentalFeature("StrictConcurrency"),
-                .enableExperimentalFeature("AccessLevelOnImport"),
-                .enableExperimentalFeature("InternalImportsByDefault"),
+                .enableUpcomingFeature("StrictConcurrency"),
+                .enableUpcomingFeature("InternalImportsByDefault"),
                 .unsafeFlags(["-static"]),
             ]
         ),
@@ -207,8 +205,7 @@ let package = Package(
             ],
             exclude: ["CMakeLists.txt", "Vendor/README.md"],
             swiftSettings: [
-                .enableExperimentalFeature("StrictConcurrency"),
-                .enableExperimentalFeature("AccessLevelOnImport"),
+                .enableUpcomingFeature("StrictConcurrency"),
                 .unsafeFlags(["-static"]),
             ]
         ),
@@ -582,7 +579,7 @@ let package = Package(
                 .product(name: "Crypto", package: "swift-crypto"),
             ],
             swiftSettings: [
-                .enableExperimentalFeature("StrictConcurrency=complete"),
+                .enableUpcomingFeature("StrictConcurrency"),
                 .unsafeFlags(["-static"]),
             ]
         ),

Sources/_AsyncFileSystem/AsyncFileSystem.swift

@@ -11,8 +11,8 @@
 //===----------------------------------------------------------------------===//
 
 import protocol _Concurrency.Actor
-@preconcurrency import struct SystemPackage.Errno
-@preconcurrency import struct SystemPackage.FilePath
+@preconcurrency package import struct SystemPackage.Errno
+@preconcurrency package import struct SystemPackage.FilePath
 
 /// An abstract file system protocol with first-class support for Swift Concurrency.
 package protocol AsyncFileSystem: Actor {

Sources/_AsyncFileSystem/MockFileSystem.swift

@@ -10,7 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-@preconcurrency import struct SystemPackage.FilePath
+@preconcurrency package import struct SystemPackage.FilePath
 
 /// In-memory implementation of `AsyncFileSystem` for mocking and testing purposes.
 package actor MockFileSystem: AsyncFileSystem {

Sources/_AsyncFileSystem/OpenReadableFile.swift

@@ -11,7 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 internal import class Dispatch.DispatchQueue
-import struct SystemPackage.FileDescriptor
+internal import struct SystemPackage.FileDescriptor
 
 /// A read-only thread-safe handle to an open file.
 package struct OpenReadableFile: Sendable {

Sources/_AsyncFileSystem/OpenWritableFile.swift

@@ -11,8 +11,8 @@
 //===----------------------------------------------------------------------===//
 
 internal import class Dispatch.DispatchQueue
-@preconcurrency import struct SystemPackage.FileDescriptor
-import struct SystemPackage.FilePath
+@preconcurrency internal import struct SystemPackage.FileDescriptor
+internal import struct SystemPackage.FilePath
 
 /// A write-only thread-safe handle to an open file.
 package actor OpenWritableFile: WritableStream {

Sources/_AsyncFileSystem/ReadableFileStream.swift

@@ -11,7 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 import _Concurrency
-import SystemPackage
+internal import SystemPackage
 internal import class Dispatch.DispatchQueue
 
 /// Type-erasure wrapper over underlying file system readable streams.