* 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.

tests/lang.sh

@@ -40,7 +40,7 @@ for i in lang/eval-okay-*.nix; do
         if test -e lang/$i.flags; then
             flags=$(cat lang/$i.flags)
         fi
-        if ! $nixinstantiate $flags --eval-only --strict lang/$i.nix > lang/$i.out; then
+        if ! NIX_PATH=lang/dir3:lang/dir4 $nixinstantiate $flags --eval-only --strict lang/$i.nix > lang/$i.out; then
             echo "FAIL: $i should evaluate"
             fail=1
         elif ! diff lang/$i.out lang/$i.exp; then

tests/lang/dir1/a.nix

@@ -0,0 +1 @@
+"a"

tests/lang/dir2/a.nix

@@ -0,0 +1 @@
+"X"

tests/lang/dir2/b.nix

@@ -0,0 +1 @@
+"b"

tests/lang/dir3/a.nix

@@ -0,0 +1 @@
+"X"

tests/lang/dir3/b.nix

@@ -0,0 +1 @@
+"X"

tests/lang/dir3/c.nix

@@ -0,0 +1 @@
+"c"

tests/lang/dir4/a.nix

@@ -0,0 +1 @@
+"X"

tests/lang/dir4/c.nix

@@ -0,0 +1 @@
+"X"

tests/lang/eval-okay-search-path.exp

@@ -0,0 +1 @@
+"abc"

tests/lang/eval-okay-search-path.flags

@@ -0,0 +1 @@
+-I lang/dir1 -I lang/dir2

tests/lang/eval-okay-search-path.nix

@@ -0,0 +1,3 @@
+import <a.nix> + import <b.nix> + import <c.nix>
+
+

tests/lang/eval-okay-search-path.nix~

@@ -0,0 +1 @@
+(import <a.nix>)

tests/lang/eval-okay-search-path.out

@@ -0,0 +1 @@
+"abc"