[Downloader] Add a command to redownload installed cog dependencies

[Drapersniper]

Feature request

Select the type of feature you are requesting:

• Cog
• Command
• API functionality

Describe your requested feature

Add a core command to be able to reinstall cog dependencies, be this to update existing dependencies or Download missing ones, this would be applicable for when people restore their bot having a command that can get all missing dependencies would make the end user life easier

an example of how this is applicable is changing python version (Going from 3.7 to 3.8 or 3.8 to 3.7) would also allow increasing the minimum python version significantly easier as you could just run this command after upgrading python to get all cog dependencies.