* 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/libexpr/common-opts.cc

@@ -33,5 +33,15 @@ bool parseOptionArg(const string & arg, Strings::iterator & i,
     return true;
 }
 
- 
+
+bool parseSearchPathArg(const string & arg, Strings::iterator & i,
+    const Strings::iterator & argsEnd, EvalState & state)
+{
+    if (arg != "-I") return false;
+    if (i == argsEnd) throw UsageError(format("`%1%' requires an argument") % arg);;
+    state.addToSearchPath(*i++);
+    return true;
+}
+
+
 }