Fix DuplicateRoot handling of subtrees

There was a bug where the `DuplicateRoot` linter would suggest merging
rules declared within nested subtrees with root rules, e.g.

    p { ... }

    @include some-mixin {
      p { ... } // This gets flagged as a duplicate of above
    }

Fix this by adding stub implementations for the various subtree node
types so we don't recurse into them.

Change-Id: I7c55cae3949d85dfa306f7cf2f65fe66b292d262
Reviewed-on: http://gerrit.causes.com/37419
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>