Move /src to /subprojects

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

subprojects/libcmd/legacy.hh

@@ -0,0 +1,24 @@
+#pragma once
+///@file
+
+#include <functional>
+#include <map>
+#include <string>
+
+namespace nix {
+
+typedef std::function<void(int, char * *)> MainFunction;
+
+struct RegisterLegacyCommand
+{
+    typedef std::map<std::string, MainFunction> Commands;
+    static Commands * commands;
+
+    RegisterLegacyCommand(const std::string & name, MainFunction fun)
+    {
+        if (!commands) commands = new Commands;
+        (*commands)[name] = fun;
+    }
+};
+
+}