Improve `aiken packages` / `aiken add` commands

[KtorZ]

What is your idea? Provide a use case.

Aiken provides commands for adding dependencies to a project (i.e. automatically edit aiken.toml). However, commands are a bit "clunky" at the moment and could be improved in the following ways:

• [ ] The --version option on aiken add could be made optional; when not provided, the dependency will be added considering only the default branch
• [ ] aiken add should verify that a given repository / project exists when adding it. Ideally, it should also check that a commit or branch given as --version also exists
• [ ] Add a aiken remove command to easily remove a dependency
• [ ] aiken package upgrade should fail with a proper error when trying to upgrade a non-existing package
• [ ] Support more providers than Github

Why is it a good idea?

These commands are seldom used at this stage because we don't yet have a large ecosystem and the stdlib comes pre-defined as dependency when running aiken new. However, the developer experience could still be made slightly better in anticipation and to prevent mistakes.

What is the current alternative and why is it not good enough?

Current alternative is not doing mistakes. It's very easy to mistype a branch name, and catching errors early is usually how we achieve good UX.