Pkg3: allow upgrading and removing packages from REPL

> When the user wants to remove package or update packages from an environment, they will instead invoke an external package management mode (julia --pkg?), which makes it clear that changes will not affect any currently running Julia sessions. The impact on usability is a strict improvement:
> - Adding packages and loading them is easier since one simply does using XYZ and answers interactive prompts.
> - Removing and upgrading packages is no less difficult since it previously required restarting the current Julia process anyway, and is less confusing since the requirement to restart is explict since running a separate process clearly doesn't affect the current one.

Using the command line to manage packages sounds like a usability loss to me. This will particularly be the case for Windows users.

I think we should keep the current `Pkg.*` functions. They should simply print a warning or ask to restart when modified packages are currently loaded.