Allow builtins.pathExists to check the existence of /nix/store paths

This makes it consitent with builtins.readDir.

src/libexpr/primops.cc

@@ -832,8 +832,14 @@ static void prim_pathExists(EvalState & state, const Pos & pos, Value * * args,
 {
     PathSet context;
     Path path = state.coerceToPath(pos, *args[0], context);
-    if (!context.empty())
-        throw EvalError(format("string '%1%' cannot refer to other paths, at %2%") % path % pos);
+    try {
+        state.realiseContext(context);
+    } catch (InvalidPathError & e) {
+        throw EvalError(format(
+                "cannot check the existence of '%1%', since path '%2%' is not valid, at %3%")
+            % path % e.path % pos);
+    }
+
     try {
         mkBool(v, pathExists(state.checkSourcePath(path)));
     } catch (SysError & e) {

tests/import-derivation.nix

@@ -10,7 +10,10 @@ let
       '';
   };
 
-  value = import bar;
+  value =
+    # Test that pathExists can check the existence of /nix/store paths
+    assert builtins.pathExists bar;
+    import bar;
 
 in