Move /src to /subprojects

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

subprojects/libstore/builtins.hh

@@ -0,0 +1,19 @@
+#pragma once
+///@file
+
+#include "derivations.hh"
+
+namespace nix {
+
+// TODO: make pluggable.
+void builtinFetchurl(
+    const BasicDerivation & drv,
+    const std::map<std::string, Path> & outputs,
+    const std::string & netrcData,
+    const std::string & caFileData);
+
+void builtinUnpackChannel(
+    const BasicDerivation & drv,
+    const std::map<std::string, Path> & outputs);
+
+}