Fixed a bug where url() statements in less modules were not correctly resolved

I had this situation:

```css
/* main.less */
@import "someModule";
```

```css
/* someModule.less */
body {
    background-image: url(../assets/img.jpg)
}
```

The problem was that when `main.less` and `someModule.less` where in the same directory this line

 ```javascript
newFileInfo.rootpath = "./" + path.relative(rootContext, newFileInfo.currentDirectory) + "/";
```

resulted in `newFileInfo.rootpath === ".//"` which is obviously wrong.