Reduce duplication in client routes map

https://github.com/novemberborn/common-path-prefix/blob/master/index.js

If multiple routes have the same route module, we can avoid duplication in client routes map.

```ts
// Before
export default {
"default": "/assets/entry.8ae2ce2d.js",
"/": "/assets/entry.8ae2ce2d.js",
"/about": "/assets/entry.ba4aa235.js",
}

// After
import {buildRouteMap} from '/@id/saus/src/client/helpers.ts'
export default buildRouteMap({
"/assets/entry.": {
"8ae2ce2d.js": ["default", "/"],
"ba4aa235.js": ["/about"],
},
})
```

The exported object would look the same as before, but the paths would be deduplicated.

**Q:** Does gzip make this a non-problem?