[Pitch] Allowing Multiple Major Versions of a Package

This pitch is based on some investigative and prototype work that I've developed over the last few months. The goal with this feature is to extend the versatility of SwiftPM as a sophisticated package manager tool that grants users the flexibility to migrate alongside major version bumps to their dependencies without the strain of being limited to a single version per package. I've brought this topic up for discussion some months back (Multiple Major Versions of a Package in SwiftPM: Opening the...