Clean up the backups package.

This is intended as an idempotent change in preparation for further patches.
I realise this is a copy&paste, but talk of getFilesToBackup is out of place. This is an exported function, and getFilesToBackup is an implementation detail that we should not be discussing.

Something like "... which  contains all the files necessary to restore the Juju agent on another machine." would be more appropriate, I think.