Fix more InternalImportsByDefault errors.

With the upcoming feature `InternalImportsByDefault` enabled, some versions of
the Swift compiler diagnose this pattern:

```
import protocol _Concurrency.Actor

// error: package protocol cannot refine an internal protocol
package protocol Proto: Actor {
  // ...
}
```

Address these diagnostics by importing `_Concurrency` with package access
level.

Author: tshortli

Sources/_AsyncFileSystem/AsyncFileSystem.swift

@@ -10,7 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-import protocol _Concurrency.Actor
+package import protocol _Concurrency.Actor
 @preconcurrency package import struct SystemPackage.Errno
 @preconcurrency package import struct SystemPackage.FilePath
 

Sources/_AsyncFileSystem/WritableStream.swift

@@ -8,7 +8,7 @@
  See http://swift.org/CONTRIBUTORS.txt for Swift project authors
 */
 
-import protocol _Concurrency.Actor
+package import protocol _Concurrency.Actor
 
 /// An asynchronous output byte stream.
 ///