Fixed bug with `resolvePath` of `project-config`

Метод `resolvePath` из модуля `project-config.js` изменяет переданный ему объект.

Например, если мы напишем что-то подобное в конфиге `make.js`:
```js
var commonLevels = [
        { path: 'libs/bem-bl/blocks-common', check: false },
        'common.blocks'
    ];

function getCommonLevels(config) {
    return commonLevels.map(function(level) {
        return config.resolvePath(level);
    });
}
```

то получим вот такую ошибку:
```console
Error: ENAMETOOLONG, name too long '/Users/blond/Projects/bem-components/
/Users/blond/Projects/bem-components//Users/blond/Projects/bem-components/
/Users/blond/Projects/bem-components//Users/blond/Projects/bem-components

...

//Users/blond/Projects/bem-components/libs/bem-bl/blocks-common'
    at Object.fs.readdirSync (fs.js:654:18)
    at module.exports.inherit.load (/Users/blond/Projects/bem-components/node_modules/enb/lib/levels/level.js:221:28)
    at /Users/blond/Projects/bem-components/node_modules/enb/techs/levels.js:91:38
    at Array.map (native)
    at /Users/blond/Projects/bem-components/node_modules/enb/techs/levels.js:90:46
    at Object.cb [as oncomplete] (fs.js:168:19)
make: *** [build] Error 1
```