When running e.g. `nixos-option users.users.ma27`, the evaluation breaks
since `ma27` is the attribute name in `attrsOf (submodule {})`, but not
a part of the option tree and therefore breaks with the following
errors:

```
error: At 'ma27' in path 'users.users.ma27': Attribute not found
An error occurred while looking for attribute names. Are you sure that 'users.users.ma27' exists?
```

This happens since the option evaluator expects that either the option
exists or the option is a submodule and the "next" token in the
attribute path points to an option (e.g. `users.users.ma27.createHome`).

This patch checks in the `Attribute not found` condition if the attribute-path
actually exists in the config tree. If that's true, a dummy-attrset is created
which contains `{_type = "__nixos-option-submodule-attr";}`, in that case, the
entire entry of the submodule will be displayed.

1. This makes aggregates of submodules (including the very important
"nixos-option users.users.<username>" case) behave the same way as any
other you-need-to-keep-typing-to-get-to-an-option-leaf (eg:
"nixos-option environment").

Before e0780c5:

  $ nixos-option users.users.root
  error: At 'root' in path 'users.users.root': Attribute not found
  An error occurred while looking for attribute names. Are you sure that 'users.users.root' exists?

After e0780c5 but before this change, this query just printed out a raw
thing, which is behavior that belongs in "nix eval", "nix-instantiate
--eval", or "nix repl <<<":

  $ nixos-option users.users.root
  {
    _module = {
      args = { name = "root"; };
      check = true;
    };
    createHome = false;
    cryptHomeLuks = null;
    description = "System administrator";
    ...

After this change:

  $ nixos-option users.users.root
  This attribute set contains:
  createHome
  cryptHomeLuks
  description
  extraGroups
  group
  hashedPassword
  ...

2. For aggregates of other types (not submodules), print out the option
that contains them rather than printing an error message.

Before:

  $ nixos-option environment.shellAliases.l
  error: At 'l' in path 'environment.shellAliases.l': Attribute not found
  An error occurred while looking for attribute names. Are you sure that 'environment.shellAliases.l' exists?

After:

  $ nixos-option environment.shellAliases.l
  Note: showing environment.shellAliases instead of environment.shellAliases.l
  Value:
  {
    l = "ls -alh";
    ll = "ls -l";
    ls = "ls --color=tty";
  }
  ...

nixos/nixos-option: fix evaluator to render a full submodule entry