Pugで別ファイルの変数が呼び出せない

Pugで「変数用のファイル」と「メインのファイル」があり、
「変数用のファイル」で定義した変数を「メインのファイル」で呼び出そうとすると、
出力されたhtmlでは値が空の状態になってしまいます。
（エラーは出ません）

特に複雑なことはしていません。

なにが原因でしょうか？

pug（2ファイルとも同階層）

//- _config.pug（変数用ファイル）
- var hoge = 'title';


//- index.pug（メインファイル）
p #{hoge}

webpack.config.js

一応、webpack関係も記載しておきます。

const path = require('path');
const globule = require('globule');
const HtmlWebpackPlugin = require('html-webpack-plugin');
const FixStyleOnlyEntriesPlugin = require('webpack-fix-style-only-entries');
const MiniCssExtractPlugin = require('mini-css-extract-plugin');
const OptimizeCSSAssetsPlugin = require('optimize-css-assets-webpack-plugin');
const TerserPlugin = require('terser-webpack-plugin');

const dir = {
  src: path.join(__dirname, '_src'),
  public: path.join(__dirname, 'public'),
  assets: 'assets',
};

const from = 'pug';
const to = 'html';
const htmlPluginConfig = globule.find([`**/*.${from}`, `!**/_*.${from}`], {
  cwd: dir.src
}).map(filename => {
  const file = filename.replace(new RegExp(`.${from}$`, 'i'), `.${to}`).split('/')
  return new HtmlWebpackPlugin({
    filename: filename.replace(new RegExp(`.${from}$`, 'i'), `.${to}`).replace(/(./)?pug/, '.'),
    template: `./${filename}`,
    inject: false,
    minify: false,
  })
});

module.exports = {
  context: dir.src,

  entry: {
    application: path.resolve(dir.src, 'js/_index.js'),
    'style.css': path.resolve(dir.src, 'sass/_index.sass'),
  },

  output: {
    filename: `${dir.assets}/js/[name].js`,
    path: dir.public
  },

  devServer: {
    contentBase: dir.public,
    open: true,
    port: 3080,
  },

  plugins: [
    new FixStyleOnlyEntriesPlugin(),
    new MiniCssExtractPlugin({
      filename: `${dir.assets}/css/[name]`
    }),
    ...htmlPluginConfig
  ],

  resolve: {
    modules: ['node_modules'],
    extensions: ['.js', '.css']
  },

  devtool: 'source-map',

  optimization: {
    minimizer: [
      new OptimizeCSSAssetsPlugin(),
      new TerserPlugin({
        terserOptions: {
          ecma: 6,
          compress: true,
          output: {
            comments: false,
            beautify: false
          }
        }
      })
    ],
  },

  module: {
    rules: [{
        test: /.js$/,
        exclude: /node_modules/,
        use: [{
          loader: 'babel-loader',
          options: {
            presets: [
              '@babel/preset-env'
            ]
          }
        }]
      },
      {
        test: /.(sass|scss|css)$/,
        use: [{
            loader: MiniCssExtractPlugin.loader
          },
          {
            loader: 'css-loader',
            options: {
              url: false,
              sourceMap: true,
              importLoaders: 2
            },
          },
          {
            loader: 'postcss-loader',
            options: {
              sourceMap: true,
              plugins: [
                require('autoprefixer')({
                  grid: true
                })
              ]
            }
          },
          {
            loader: 'sass-loader',
            options: {
              sourceMap: true
            }
          },
        ],
      },
      {
        test: /.pug$/,
        use: {
          loader: 'pug-loader',
          options: {
            pretty: true
          }
        }
      }
    ]
  }
};

package.json

{
  "name": "webpack",
  "version": "1.0.0",
  "private": true,
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
    "watch": "webpack --watch --inline --mode=development",
    "start": "webpack-dev-server",
    "build": "webpack --mode=production"
  },
  "devDependencies": {
    "@babel/core": "^7.11.0",
    "@babel/preset-env": "^7.11.0",
    "autoprefixer": "^9.8.6",
    "babel-loader": "^8.1.0",
    "css-loader": "^3.6.0",
    "globule": "^1.3.2",
    "html-webpack-plugin": "^4.3.0",
    "mini-css-extract-plugin": "^0.9.0",
    "optimize-css-assets-webpack-plugin": "^5.0.3",
    "postcss-loader": "^3.0.0",
    "pug": "^2.0.3",
    "pug-loader": "^2.4.0",
    "sass": "^1.26.10",
    "sass-loader": "^9.0.2",
    "terser-webpack-plugin": "^3.1.0",
    "webpack": "^4.43.0",
    "webpack-cli": "^3.3.12",
    "webpack-dev-server": "^3.11.0",
    "webpack-fix-style-only-entries": "^0.5.1"
  },
  "browserslist": [
    "last 2 version",
    "> 1%"
  ]
}