Allow renaming a package

It aught to be rare, but there are times when renaming a package is desirable - such as when an author manages to use someone elses namespace; make it possible to rename a package without deleting and recreating the package.

A follow up to this could be to track renames and automatically handle/redirect requests for the renamed repository - though that's a bit more involved and not something I wish to implement at this time (IMO the way the search handles requests for missing packages is not perfect, though sufficient).

This resolves #47 and would help alleviate #163