Handle bitbucket deletion and redirection

Many repos were deleted when bitbucket dropped Mercurial support in 2020. See https://wiki.archiveteam.org/index.php/Bitbucket

https://bitbucket.org/pypa/distlib/ may be one of them. If this displayed consistently, we could follow the URL, and create a PURL "alias" in the DB?

```
This repository has been deleted

Our apologies, but the repository "pypa/distlib" has been deleted.

It now lives at https://github.com/pypa/distlib.
```

Also FWIW, some old versions were collected in SoftwareHeritage like for here https://bitbucket-archive.softwareheritage.org/new-static/e1/e1d6b725-3b7d-4c7a-8d3c-3f1ee73c96fd/attachments/
Or saved at https://archive.org/details/archiveteam_bitbucket