Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

src/libutil/windows/windows-async-pipe.hh

@@ -1,27 +0,0 @@
-#pragma once
-///@file
-
-#include "file-descriptor.hh"
-
-namespace nix::windows {
-
-/***
- * An "async pipe" is a pipe that supports I/O Completion Ports so
- * multiple pipes can be listened too.
- *
- * Unfortunately, only named pipes support that on windows, so we use
- * those with randomized temp file names.
- */
-class AsyncPipe
-{
-public:
-    AutoCloseFD writeSide, readSide;
-    OVERLAPPED overlapped;
-    DWORD got;
-    std::vector<unsigned char> buffer;
-
-    void createAsyncPipe(HANDLE iocp);
-    void close();
-};
-
-}