Shell completion should complete ebuild and repository names

[vincentdephily]

The emlop binary could help in generating possible names (UI would be to add another argument after emlop complete <shell> <partial string>), but we still need to hook that into the shell completion generated by clap.

[vincentdephily]

Also, as a regression from the config file support work, many "enum" flags no longer complete possible variants. Eg --resume is still clap-parsed and completes main/backup/etc but --duration is now emlop-parsed and just completes file names.

I've started fixing this in the complete branch.

[vincentdephily]

Branch merged.

For zsh, package name only matches from the start of the string. Maybe some knowledgeable user can do better than me.