normalizeConfig excludes dependencies from build

[johannes-z]

See: https://github.com/egoist/bili/blob/master/src/index.ts#L126

Even with bundleNodeModules for umd-modules, all dependencies specified in package.json are externalized. This is the only way I found to make externals work properly:

  extendConfig: config => {
    config.externals = [
      'myExternals...'
    ]
    return config
  },

[djkloop]

same issue #559