* 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/eval.hh

@@ -213,11 +213,16 @@ private:
 
     std::map<Path, Expr *> parseTrees;
 
+    Paths searchPath;
+    Paths::iterator searchPathInsertionPoint;
+
 public:
     
     EvalState();
     ~EvalState();
 
+    void addToSearchPath(const string & s);
+
     /* Parse a Nix expression from the specified file.  If `path'
        refers to a directory, then "/default.nix" is appended. */
     Expr * parseExprFromFile(Path path);
@@ -229,6 +234,9 @@ public:
        form. */
     void evalFile(const Path & path, Value & v);
 
+    /* Look up a file in the search path. */
+    Path findFile(const string & path);
+
     /* Evaluate an expression to normal form, storing the result in
        value `v'. */
     void eval(Expr * e, Value & v);