Move /src to /subprojects

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

subprojects/libstore/daemon.hh

@@ -0,0 +1,18 @@
+#pragma once
+///@file
+
+#include "serialise.hh"
+#include "store-api.hh"
+
+namespace nix::daemon {
+
+enum RecursiveFlag : bool { NotRecursive = false, Recursive = true };
+
+void processConnection(
+    ref<Store> store,
+    FdSource && from,
+    FdSink && to,
+    TrustedFlag trusted,
+    RecursiveFlag recursive);
+
+}