Move /src to /subprojects

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

subprojects/libstore/unix/build/hook-instance.hh

@@ -0,0 +1,41 @@
+#pragma once
+///@file
+
+#include "logging.hh"
+#include "serialise.hh"
+#include "processes.hh"
+
+namespace nix {
+
+struct HookInstance
+{
+    /**
+     * Pipes for talking to the build hook.
+     */
+    Pipe toHook;
+
+    /**
+     * Pipe for the hook's standard output/error.
+     */
+    Pipe fromHook;
+
+    /**
+     * Pipe for the builder's standard output/error.
+     */
+    Pipe builderOut;
+
+    /**
+     * The process ID of the hook.
+     */
+    Pid pid;
+
+    FdSink sink;
+
+    std::map<ActivityId, Activity> activities;
+
+    HookInstance();
+
+    ~HookInstance();
+};
+
+}