Move /src to /subprojects

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

subprojects/libutil/windows/file-system.cc

@@ -0,0 +1,17 @@
+#include "file-system.hh"
+
+namespace nix {
+
+Descriptor openDirectory(const std::filesystem::path & path)
+{
+    return CreateFileW(
+        path.c_str(),
+        GENERIC_READ,
+        FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,
+        NULL,
+        OPEN_EXISTING,
+        FILE_FLAG_BACKUP_SEMANTICS,
+        NULL);
+}
+
+}