EvalState::realiseContext(): Allow access to the entire closure

Fixes #11030.

(cherry picked from commit 08361f031d)

# Conflicts:
#	src/libexpr/eval.cc
#	tests/functional/import-from-derivation.nix
#	tests/functional/import-from-derivation.sh

tests/functional/import-from-derivation.nix

@@ -0,0 +1,52 @@
+with import <config>;
+
+rec {
+  bar = mkDerivation {
+    name = "bar";
+    builder = builtins.toFile "builder.sh"
+      ''
+        echo 'builtins.add 123 456' > $out
+      '';
+  };
+
+  value =
+    # Test that pathExists can check the existence of /nix/store paths
+    assert builtins.pathExists bar;
+    import bar;
+
+  result = mkDerivation {
+    name = "foo";
+    builder = builtins.toFile "builder.sh"
+      ''
+        echo -n FOO${toString value} > $out
+      '';
+  };
+
+  addPath = mkDerivation {
+    name = "add-path";
+    src = builtins.filterSource (path: type: true) result;
+    builder = builtins.toFile "builder.sh"
+      ''
+        echo -n BLA$(cat $src) > $out
+      '';
+  };
+
+  step1 = mkDerivation {
+    name = "step1";
+    buildCommand = ''
+      mkdir -p $out
+      echo 'foo' > $out/bla
+    '';
+  };
+
+  addPathExpr = mkDerivation {
+    name = "add-path";
+    inherit step1;
+    buildCommand = ''
+      mkdir -p $out
+      echo "builtins.path { path = \"$step1\"; sha256 = \"7ptL+pnrZXnSa5hwwB+2SXTLkcSb5264WGGokN8OXto=\"; }" > $out/default.nix
+    '';
+  };
+
+  importAddPathExpr = import addPathExpr;
+}