fix nested directory imports

This fixes a problem where the `index.less` file associated to an
`@import './directory'` statement couldn't be resolved if you had more
than one level of nested directories, like this:

    .
    ├── index.less
    └── moduleA
        ├── index.less
        └── moduleB
            ├── index.less
            └── moduleC
                └── index.less

    // index.less
    @import './moduleA';

    // moduleA/index.less
    import './moduleB';

    // moduleA/moduleB/index.less
    import './moduleC';