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/src/libexpr/lexer.l

@@ -14,8 +14,8 @@
 %{
 #include <boost/lexical_cast.hpp>
 
-#include "nixexpr.hh"
-#include "parser-tab.hh"
+#include "libexpr/nixexpr.hh"
+#include "generated/parser-tab.hh"
 
 using namespace nix;