错误：找不到模块html-webpack-plugin

Question

我正在处理Webpack上的教程，但在添加HtmlWebpackPlugin并尝试再次运行Webpack之后，我收到以下错误：

Error: Cannot find module 'html-webpack-plugin'
    at Function.Module._resolveFilename (module.js:489:15)
...

我已尝试在本地和全局安装插件，但我两次都收到错误。

//webpack.config.js
const path = require('path');
const HtmlWebpackPlugin = require('html-webpack-plugin');

module.exports = {
  entry: "./app.js", // bundle entry point
  output: {
    path: path.resolve(__dirname, 'dist'), // output directory
    filename: "[name].js" // name of generated bundle
  },
  plugins : [
    new HtmlWebpackPlugin({
      template: "index.html",
      inject: "body"
    })
  ]
};

//package.json
{
  "name": "hyperlocalnola",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "html-webpack-plugin": "^2.30.1"
  }
}

//app.js
var msg = require("./contents.js");
document.write(msg);

//contents.js
module.exports = "hello friend!";

对于可能非常简单的文本墙感到抱歉，但我无法找到导致此问题的原因。提前谢谢。