Fix InternalImportsByDefault diagnostics in _AsyncFileSystem (#7895)

Address some `InternalImportsByDefault` diagnostics in the
`_AsyncFileSystem` which have crept in because the Swift 6 toolchain
isn't enforcing
`.enableExperimentalFeature("InternalImportsByDefault")`.

### Motivation:

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.


### Modifications:

Adjusted access level on a number of import statements in files
belonging to the `_AsyncFileSystem` target.

### Result:

`_AsyncFileSystem` builds successfully when `InternalImportsByDefault`
is actually enabled with the Swift 6 toolchain.

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.