Multiple sitemap.txt for packages

The current sitemap handler has the following comment:

```
// Google wants the return page to have < 50,000 entries and be less than
// 50MB - https://support.google.com/webmasters/answer/183668?hl=en
// As of 2018-01-01, the return page is ~3,000 entries and ~140KB
// By restricting to packages that have been updated in the last two years,
// the count is closer to ~1,500
```

As of today, it has 12581 entries, its size is ~567K. We should probably shard it by the first letter of the package name or something similar.