If there is only one file for named entry, pass it directly

[jakub300]

Hey,

Currently webpack-steam always passes files for named entry as an array. Because of that webpack is creating dummy module and all it does is requiring our main file, for example:

/***/ 4:
/***/ (function(module, exports, __webpack_require__) {

module.exports = __webpack_require__(5);


/***/ }),

It is ugly and what's worse that module is does not receive id from HashedModuleIdsPlugin (that's what lead me to discovery of current behaviour). This PR fixes it by extracting first element from the array if the array has only one element.