Add implicit dependency on original module during dedupe

This change ensures the original module is recorded as a dependency of a deduplicated module.

When modules are deduplicated and dependencies between the duplicate and original are the same, the resulting duplicate module code looks like this:

```js
module.exports=require(<original-module-index>)
```

Arguably, this implies that there is now a dependency on the original module from the duplicate, but this information was not being recorded.

Previously, plugins that use dependency information (i.e., factor-bundle) were getting an incomplete picture of a row's dependencies if it had been deduplicated. This was causing issues such as substack/factor-bundle#38 and substack/factor-bundle#39.