Is your feature request related to a problem? Please describe.
At this point in time if you want to edit an existing package, you have to export it, open its config in your favorite text editor, edit the config, save it; and in the case that you actually want to replace the old package config with the new one, you have to remove the old package and import the new one manually.
It sounds worse than it actually is, yet still it's quite a lot to do for a simple task.

Describe the solution you'd like
A simpler approach would be to add something like a subcommand proji package edit LABEL and let proji automatically open the config file in your favorite text editor. Proji would copy the original config to a temp or cache folder, open this copy for editing and check for changes, if the file has changed, ask the user if he wants to replace the old config with it.

Additional context
Obviously that would make the editing process more enjoyable but there a lot of things which might not really work out as planned. I guess I'll implement it for tryouts and we'll see.