This example demonstrates what I believe is an issue with using var externals in a UMD build.

I have added two externals: one for lodash and a custom var external as follows:

module.exports = {
  /* ... */
  externals: {
    lodash: {
      commonjs: 'lodash',
      commonjs2: 'lodash',
      amd: 'lodash',
      root: '_'
    },
    foobar: 'var foo.bar'
  }
};

The output bundle contains things like require("foo.bar"), define(["lodash", "foo.bar"], factory), and the foo.bar external has a corresponding __WEBPACK_EXTERNAL_MODULE_2__. However, upon inspecting the bundle, you can see that __WEBPACK_EXTERNAL_MODULE_2__ is in fact unused, and there is a different module that is used and simply exports

/* 2 */
/***/ (function(module, exports) {

module.exports = foo.bar;

/***/ })

This makes we wonder if it's possible to not output WEBPACK_EXTERNAL_MODULE_2__ as that would solve the problem with the invalid (as well as unused) require("foo.bar").