Fix InternalImportsByDefault diagnostics in _AsyncFileSystem.

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 with newer toolchains, 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.

Author: tshortli

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.