* Add a Nix expression search path feature. Paths between angle

brackets, e.g.

    import <nixpkgs/pkgs/lib>

  are resolved by looking them up relative to the elements listed in
  the search path.  This allows us to get rid of hacks like

    import "${builtins.getEnv "NIXPKGS_ALL"}/pkgs/lib"

  The search path can be specified through the ‘-I’ command-line flag
  and through the colon-separated ‘NIX_PATH’ environment variable,
  e.g.,

    $ nix-build -I /etc/nixos ...

  If a file is not found in the search path, an error message is
  lazily thrown.

src/nix-instantiate/nix-instantiate.cc

@@ -107,6 +107,8 @@ void run(Strings args)
         }
         else if (parseOptionArg(arg, i, args.end(), state, autoArgs))
             ;
+        else if (parseSearchPathArg(arg, i, args.end(), state))
+            ;
         else if (arg == "--add-root") {
             if (i == args.end())
                 throw UsageError("`--add-root' requires an argument");