[Feature] Create a package subcommand in leo CLI

[damirka]

🚀 Feature

I suggest adding leo package subcommand. So Aleo PM related commands would look like:

leo package add <package>
leo package remove <package>
leo package login ...
leo package publish
leo package search ... 
# etc

If this looks long, we can shorten package to pkg or think of a different naming.

Motivation

We already have many and will be having even more commands within leo CLI. Also some commands are API heavy (and involve logging in) and some only use algorithms. By splitting commands into smaller groups we help understanding them by first look and also split logic within the CLI application.

So the enduser clearly sees which commands may require Aleo PM credentials and which can be run locally.

Implementation

Subgrouping can be easily implemented within #584 if it receives a green light due to StructOpt flatten feature.

[damons]

This makes sense to me.

[damirka]

Won't do due to this comment.