fix: Set https urls for packages

[facumenzella]

Motivation

This is a follow-up of #4661

• The url keeps changing from ssh to https.
• Tweak how nimble is declared as a dependency

[ajpallares]

Looks good to me. However, let's have someone else take a look to make sure that they can properly resolve packages versions and doing so does not create diffs in Package.resolved for them

[facumenzella]

@RCGitBot please test