refactor(3p/nix): Anchor local includes at src/

Previously all includes were anchored in one global mess of header
files. This moves the includes into filesystem "namespaces" (if you
will) for each sub-package of Nix.

Note: This commit does not introduce the relevant build system changes.

third_party/nix/tests/plugins/plugintest.cc

@@ -1,4 +1,4 @@
-#include "config.hh"
+#include "libutil/config.hh"
 #include "primops.hh"
 
 using namespace nix;