This helps with troubleshooting exceptions in config values, which were hard
to track down for options with many definitions.
The trace will look like:

    error: while evaluating the attribute 'config.foo' at undefined position:
    [...]
    while evaluating the option `foo':
    [...]
    while evaluating definitions from `/home/user/mymod.nix':
    while evaluating 'dischargeProperties' at /home/user/nixpkgs/lib/modules.nix:464:25, called from /home/user/nixpkgs/lib/modules.nix:392:137:
    while evaluating the attribute 'value' at /home/user/nixpkgs/lib/modules.nix:277:44:
    Value error!

where the `/home/user/mymod.nix` module is

    { lib, ... }: {
      options.foo = lib.mkOption {
        type = lib.types.lines;
      };
      config.foo = builtins.throw "Value error!";
    }

lib/modules.nix: Add file context to unmerged values in mergeDefinitions