Aurelia adds /index to modules path after ModuleConcatenationPlugin

**I'm submitting a bug report**

* **Library Version:**
3.0.0

**Please tell us about your environment:**
* **Operating System:**
OSX 10.13.6

* **Node Version:**
10.15.3

* **NPM Version:**
6.4.1

* **JSPM OR Webpack AND Version**
webpack 4.29.1

* **Browser:**
all

* **Language:**
ESNext

**Current behavior:**

When using `.plugin(PLATFORM.moduleName('my/plugin/path'))` on a production build, where `ModuleConcatenationPlugin` runs by default, the module ends up on the bundle with the key `my/plugin/path/index`, so the app fails to run as `my/plugin/path` doesn't exist.

**Expected/desired behavior:**

On the main `configure` of the app, use `.plugin` for a local folder index file that includes a component/service/valueConverter, basically anything that the `ModuleConcatenationPlugin` may want to optimize:

```js
// main.js

export const configure = (aurelia) => {
aurelia.use
.plugin(PLATFORM.moduleName('modules/my-module'))
.standardConfiguration();

return aurelia.start().then(() => aurelia.setRoot(...));
};

// modules/my-module/index.js

import { MY_CONSTANT } = './my-constant.js';

export const configure = (aurelia) => {
console.log(MY_CONSTANT);
};

// modules/my-module/my-constant.js

export const MY_CONSTANT = {
someKey: 'someValue',
};
```

That's enough for `ModuleConcatenationPlugin` to try to put `my-constant` inside `my-module/index.js` and for the `PreserveModuleNamePlugin` to change `modules/my-module` to `modules/my-module/index`.

* **What is the expected behavior?**

For `PreserveModuleNamePlugin` not to add the `/index`.

* **Possible solution**

Not sure if this is the right way to do it, but if we add the following code before [this line](https://github.com/aurelia/webpack-plugin/blob/master/src/PreserveModuleNamePlugin.ts#L61), it works:

```js
if (id.endsWith('/index') && !realModule.rawRequest.endsWith('index')) {
id = id.replace(/\/index$/, '');
// or id = id.substr(0, id.length - 6);
}
```
I'm not that familiar with the module structure, so I'm not sure if we can "trust" in `rawRequest`

Start with src/PreserveModuleNamePlugin.ts around line 61 and reproduce the production build using ModuleConcatenationPlugin with a local folder index module. Inspect how the module id and rawRequest are handled when the bundle key gains /index. Done means the configured module remains addressable as my/plugin/path rather than my/plugin/path/index.

Written by the indexing model from the issue text.