Replace bitbucket.org/creachadair/shell with https://github.com/creachadair/mds

Hi! I help maintain this package in Debian. Trillian has a dependency on bitbucket.org/creachadair/shell but that package says:

```
(deprecated)

This repository is no longer maintained. It remains here to avoid breaking existing imports of bitbucket.org/creachadair/shell, but users should consider switching to the new location, http://godoc.org/github.com/creachadair/mds/shell . Further improvements and fixes will be made there, not here.
```

Could you make the transition to github.com/creachadair/mds/shell so we can drop the deprecated package from Debian?

Thanks,
Simon

Start by searching the repository for imports of bitbucket.org/creachadair/shell and inspect the Go dependency declarations that reference it. Replace the deprecated package path with github.com/creachadair/mds/shell, then run the relevant Go tests or build checks to confirm the dependency transition is complete.

Written by the indexing model from the issue text.