webpack打包的ES6代碼，import獲取的值為空/undefined

webpack配置如下啦：

const webpack = require('webpack')
const path = require('path')

const config = {
    context: path.resolve(__dirname, '../src'),
    entry: '../src/app.js',
    output: {
        path: path.resolve(__dirname, '../dist'),
        filename: 'bundle.js'
    },
    module: {
        rules: [{
            test: /\.js$/,
            include: path.resolve(__dirname, '../src'),
            use: [{
                loader: 'babel-loader',
                options: {
                    presets: ['env']
                }
            }]
        }]
    },
    plugins:[
    ]
}

module.exports = config

使用如上的webpack配置，打包app.js如下：

import a from './a'
import b from './b'
export{
    a,
    b
}

打包完發(fā)布npm，另一個(gè)項(xiàng)目使用該模塊包，import {a} from xxx,卻拿不到a或者b的值分別為undefined,這是為啥？（上述打包一切正常，已使用babel轉(zhuǎn)換，依賴引入的也是轉(zhuǎn)換后的es5代碼）求指教！