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/nixexpr.hh

@@ -2,9 +2,9 @@
 
 #include <map>
 
-#include "symbol-table.hh"
-#include "types.hh"  // TODO(tazjin): audit this include
-#include "value.hh"
+#include "libexpr/symbol-table.hh"
+#include "libexpr/value.hh"
+#include "libutil/types.hh"  // TODO(tazjin): audit this include
 
 namespace nix {