_AsyncFileSystem: reduce visibility of import (NFCI)

This reduces the visibility of the import as it is only used in internal
declarations. This cleans up a recently introduced warning.

Author: compnerd

Sources/_AsyncFileSystem/ReadableFileStream.swift

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