CommonJS compatibility does not work as expected

The compiler puts each commonJs module into `default` property of the imported object.
```js
// ecma
import commonJs from './common-js'

console.log('requiring a common js from ecma:')
console.log(commonJs)
```

CommonJS From ECMACommonJS From CommonJS

```js
// common-js.js
const commonJs2 = require('./common-js2')

module.exports = () => {
console.log('default common js export')
}
module.exports['named'] = () => {
console.log('named common js export')
}

console.log('requiring a ' +
'common js from common js:')
console.log(commonJs2)
```

```js
// common-js2.js
module.exports = () => {
console.log('default common js '
+ 'export2')
}
module.exports['named'] = () => {
console.log('named common js '
+ 'export2')
}
```

Run the compiler
```fs
java -jar /Users/zavr/node_modules/google-closure-compiler-java/compiler.jar \
--compilation_level ADVANCED --language_out ECMASCRIPT_2017 --formatting PRETTY_PRINT \
--process_common_js_modules --entry_point \
example/commonjs/index.js --module_resolution NODE --js \
example/commonjs/index.js example/commonjs/common-js.js example/commonjs/common-js2.js
```

Get the output

```js
'use strict';
var a = () => {
console.log("default common js export2");
};
a.named = () => {
console.log("named common js export2");
};
var b = {default:() => {
console.log("default common js export");
}};
b.default.named = () => {
console.log("named common js export");
};
console.log("requiring a common js from common js:");
console.log(a);
console.log("requiring a common js from ecma:");
console.log(b);
```
Execute the output
```jsx
requiring a common js from common js:
{ [Function: a] named: [Function] }
requiring a common js from ecma:
{ default: { [Function: default] named: [Function] } }
```
This does not make sense. I shouldn't write `.default` as confirmed here https://github.com/google/closure-compiler/issues/3308. When it comes to Babel-compiled modules, it's a nightmare. Babel exports { default: code } itself, so the currently closure-compatible code becomes

```js
import erte from '@a-la/fixture-babel'

console.log(erte.default.default())
console.log(erte.default.c())
console.log(erte.default.b())
```

And if you actually try to execute in with babel, you get
```
console.log(_.default.default.default());
^

TypeError: Cannot read property 'default' of undefined
```

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